Choosing the wrong steel post is the most common structural mistake in retaining wall construction. Undersized posts deflect under load, causing sleepers to bow, walls to lean, and ultimately structural failure. This guide covers every decision in post selection — series, profile, length, embedment depth, footing size, post spacing, and when your state requires engineering and council approval.
Browse the full steel post range or jump to a section:
The three post series
100 Series — standard residential
The 100 Series — 100UC14.8 H-beam and 100PFC C-channel — is the standard residential specification for walls up to approximately 1.0m in stable, well-drained soils with no significant surcharge loads.
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Suitable for: garden walls, terracing, low boundary walls in sandy or loam soils in Melbourne bayside, Sydney northern beaches, Brisbane Northside, Adelaide plains, Perth coastal suburbs
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Not suitable for: reactive clay soils, walls over 1.0m, driveway surcharge, sloping sites with heavy runoff, or walls near boundaries in most states
150 Series — heavy duty
The 150 Series — 150UC30 H-beam and 150PFC C-channel — is the heavy duty specification for walls 1.0m–1.8m, reactive clay soils, and sites with elevated surcharge loads.
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Suitable for: reactive clay soils across Melbourne's western and eastern suburbs, Western Sydney (Wianamatta Shale), Brisbane outer west, Adelaide inner and northern suburbs, and Perth hills; walls 1.0m–1.8m; driveway surcharge; coastal sites within 1km of surf
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Recommended as the minimum specification for any Australian residential site where soil conditions are unknown — it is the most common post used in trade work nationally
200 Series — engineered
The 200 Series — 200UC46 H-beam and 200PFC C-channel — is the engineered specification for walls over 1.8m, high surcharge loads, and civil or commercial applications.
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Suitable for: walls over 1.8m, heavy vehicle or crane surcharge, civil retaining walls, sites where a structural engineer has specified a 200UC or 200PFC section
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Always requires a structural engineer's specification for post length, footing design and reinforcement before ordering
| Series |
Section |
Wall height (standard soil) |
Wall height (reactive clay) |
Surcharge |
| 100 Series |
100UC14.8 / 100PFC |
Up to 1.0m |
Up to 0.8m |
Garden only |
| 150 Series |
150UC30 / 150PFC |
1.0m–1.8m |
1.0m–1.5m |
Driveway / vehicle |
| 200 Series |
200UC46 / 200PFC |
Over 1.8m |
Over 1.5m |
Heavy / structure / pool |
These are indicative ranges only. Post sizing must be confirmed by a structural engineer for all walls over 1.0m, walls in reactive soil, or walls carrying surcharge loads.
H-beam vs C-channel — which profile?
H-beam (UC — Universal Column) has a symmetrical I-section providing higher bending resistance in both the strong and weak axis. Concrete sleepers slot into both flanges. H-beam is the correct choice for:
- Any wall in reactive or expansive clay soil
- Corner posts and return posts (symmetrical loading)
- Walls over 800mm in retained height
- Walls with surcharge loads (driveways, pools, structures)
- Sites where soil classification is unknown
C-channel (PFC — Parallel Flange Channel) has an open channel section into which sleepers slot from one side. It has slightly lower bending strength than the equivalent UC section but is adequate for:
- Walls under 600mm in standard soil with garden loading only
- Fencing applications and under-fence plinths
- End posts at the termination of a wall run
- Low-load boundary walls in well-drained sandy soils
Read the full H-beam vs C-channel guide for a section-by-section comparison with worked examples.
Post length and embedment depth
Post length is calculated as: exposed wall height + embedment depth.
Embedment depth is the most commonly underspecified element in residential retaining walls. Insufficient embedment is a leading cause of post rotation and wall failure. The 1:3 rule (embedment = one-third of total post length) is widely quoted as a rule of thumb, but it is a starting point only — not a design standard.
Embedment by soil type
| Soil type |
Wall height |
Minimum embedment |
Total post length |
Notes |
| Standard (sandy loam, stable clay) |
0.6m |
600mm |
1.2m |
100 Series |
| Standard (sandy loam, stable clay) |
0.8m |
800mm |
1.6m |
100 or 150 Series |
| Standard (sandy loam, stable clay) |
1.0m |
1,000mm |
2.0m |
150 Series — engineering required |
| Standard (sandy loam, stable clay) |
1.2m |
1,200mm |
2.4m |
150 Series — engineering required |
| Reactive clay (Class H1–H2) |
0.6m |
750mm |
1.35m |
+25% over standard |
| Reactive clay (Class H1–H2) |
0.8m |
1,000mm |
1.8m |
150 Series minimum |
| Reactive clay (Class H1–H2) |
1.0m |
1,250mm |
2.25m |
Engineer specification required |
| Highly reactive / Extreme (Class E) |
Any height |
Engineer designed |
Engineer designed |
150 or 200 Series — geotechnical report recommended |
| Sandy coastal / loose fill |
0.6m |
700mm |
1.3m |
Wider footing to compensate for lower lateral resistance |
| Rock (sandstone, basalt) |
Any height |
Engineer designed |
Engineer designed |
Rock anchor or cored footing required |
Important note on AS 4678:2002 Amendment 1: The minimum live load surcharge for walls over 1.5m was raised to 5 kPa under Amendment 1. Some manufacturer span tables still reference the superseded 2.5 kPa value. Always confirm that any span table you are using has been updated to reflect Amendment 1.
Full detail by soil type and wall height: Post Embedment Depth Guide.
The footing transfers the bending moment from the post into the surrounding soil. A footing that is too narrow in diameter fails to develop adequate passive soil resistance — the post rotates rather than deflects, and the wall falls.
| Post series |
Soil type |
Minimum hole diameter |
Concrete specification |
Bags per footing (approx.) |
| 100 Series |
Standard |
300mm Ø |
Minimum 20MPa |
2–3 × 20kg bags |
| 100 Series |
Reactive clay |
350mm Ø |
Minimum 25MPa |
3–4 × 20kg bags |
| 150 Series |
Standard |
350mm Ø |
Minimum 25MPa |
3–4 × 20kg bags |
| 150 Series |
Reactive clay |
450mm Ø |
Minimum 25MPa |
5–7 × 20kg bags |
| 200 Series |
Any |
450mm Ø minimum — engineer to confirm |
Engineer specified |
Engineer specified |
Use rapid-set concrete for residential footings — it sets in 30–60 minutes, allowing backfilling the same day. Add 10–15% extra bags to account for variations in hole diameter. Do not place sleepers until footings have fully cured — minimum 48 hours for rapid-set.
Post spacing by wall height and soil type
Post spacing determines the span of concrete sleepers between posts. Longer spans mean higher bending stress on each sleeper and higher load on each post. Wider post spacing is economical (fewer posts, fewer holes) but must be matched to both the sleeper thickness and the post series.
| Wall height |
Soil type |
Max post spacing |
Recommended post |
| Up to 0.6m |
Standard |
2400mm centres |
100 Series C-channel or H-beam |
| Up to 0.8m |
Standard |
2000mm centres |
100 Series H-beam |
| Up to 1.0m |
Standard |
2000mm centres |
150 Series H-beam |
| Up to 1.0m |
Reactive clay |
1800mm centres |
150 Series H-beam |
| 1.0m–1.5m |
Standard |
1800–2000mm centres |
150 Series H-beam — engineering required |
| 1.0m–1.5m |
Reactive clay |
1600–1800mm centres |
150 Series H-beam — engineering required |
| Over 1.5m |
Any |
Engineer to specify |
150 or 200 Series — engineering required |
Surcharge loads from driveways, vehicles, structures or slopes above the wall increase the bending moment on posts significantly. When surcharge is present, reduce post spacing by one step, upgrade from C-channel to H-beam, and confirm spacing with a structural engineer. Full detail: Post Spacing Guide.
Soil types by state and city — what it means for post selection
Australian soil conditions vary more dramatically between suburbs than between cities. Getting the soil classification right before you order is the most important step in post selection.
New South Wales
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Sydney — sandstone (Eastern Suburbs, North Shore, Northern Beaches, Inner West): Excellent bearing capacity, low lateral pressure, good drainage. 100 Series adequate for most walls under 1.0m. Post holes require rock-breaking equipment in solid sandstone — factor this into installation cost before specifying depth.
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Western Sydney — Wianamatta Shale clay (Penrith, Blacktown, Parramatta, Liverpool, Campbelltown): Highly reactive Class H2 soil. 150 Series minimum for all walls; increase embedment by 25% over standard. H-beam posts for all walls regardless of height. See the Sydney retaining wall guide.
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Central Coast and Hunter Region (Newcastle): Mixed sandy loam and clay, varying by suburb. 100 or 150 Series depending on site assessment.
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Blue Mountains: Exposed sandstone with clay-filled gullies. Post specification varies significantly by exact site — geotechnical assessment recommended.
Victoria
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Melbourne western suburbs (Werribee, Hoppers Crossing, Melton, Tarneit, Wyndham Vale, Sunbury): Basalt-derived black clay, Class H2 to Class E under AS 2870. The most reactive soil type in Melbourne. 150 Series minimum; 200 Series for walls over 1.2m. Embedment minimum 1.5× wall height. 450mm footing diameter. 100mm ag pipe mandatory. See the Melbourne retaining wall guide.
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Melbourne eastern suburbs (Knox, Maroondah, Whitehorse, Yarra Ranges): Silurian clay — highly reactive, one of the most expansive in Australia. 150 Series for all walls without exception. H-beam posts only. Increase embedment by 25%.
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Melbourne inner and south-eastern suburbs (Bayside, Mornington Peninsula, Frankston): Sandy soils near the coast, transitioning to clay loam inland. 100 Series adequate for bayside sandy sites under 1.0m. Coastal sites within 1km of surf — confirm corrosion specification.
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Melbourne northern growth corridor (Craigieburn, Mickleham, Epping): Reactive clay similar to western suburbs. 150 Series minimum.
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Geelong: Mixed basalt and sedimentary soils — site-specific assessment recommended.
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Ballarat and Bendigo: Generally stable soils with some reactive clay pockets. 100 or 150 Series depending on site assessment.
Queensland
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Brisbane inner and north (New Farm, Paddington, Ashgrove, Chermside): Weathered granite and clay loam. 100 Series for low walls in stable areas; 150 Series for clay sites.
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Brisbane outer west (Ipswich, Springfield, Richlands, Inala): Reactive Ipswich clay, Class H1–H2. 150 Series minimum; H-beam posts for all walls.
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Gold Coast: Generally sandy and well-drained coastal soils; Hinterland areas have clay. 100 Series suitable for most Gold Coast residential applications under 1.0m away from the hinterland.
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Sunshine Coast (Maroochydore, Caloundra, Noosa): Sandy coastal soils transitioning to clay inland. 100 Series for coastal suburbs, 150 Series for hinterland sites.
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Cairns and Far North QLD: Wet tropics soils with high moisture and significant wet season pressure. 150 Series minimum; drainage specification is critical.
South Australia
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Adelaide plains (inner and northern suburbs — Prospect, Gepps Cross, Elizabeth): Reactive clay, Class H1–H2. 150 Series minimum; H-beam posts. See the SA regulations guide.
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Adelaide coastal (Glenelg, Brighton, Henley Beach, Semaphore): Sandy well-drained soils. 100 Series adequate for most walls under 1.0m.
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Adelaide Hills and Foothills (Stirling, Aldgate, Hahndorf): Rocky soils with clay pockets. Rock boring risk — get a geotechnical assessment before specifying post length.
Western Australia
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Perth coastal (Cottesloe, City Beach, Scarborough, Fremantle, Joondalup): Sandy, very well-drained Bassendean or Spearwood sands. 100 Series adequate for most residential walls under 1.0m. Wide footings preferred over deep footings to develop lateral resistance in loose sand.
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Perth hills (Kalamunda, Mundaring, Armadale): Laterite and clay soils. 150 Series recommended; variable conditions — site-specific assessment recommended.
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Perth northern corridor (Wanneroo, Yanchep, Two Rocks): Predominantly sandy — 100 Series for most residential applications.
Tasmania, ACT and NT
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Hobart: Dolerite rock and clay soils. Post holes in dolerite areas require rock-breaking equipment. 150 Series for clay sites.
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Canberra and ACT: Granite-derived soils with reactive clay pockets, particularly in the Gungahlin and Molonglo corridors. 150 Series for clay sites.
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Darwin and Northern Territory: Tropical conditions with significant wet season rainfall. 150 Series minimum; corrosion specification critical in the coastal zone.
Not sure of your soil classification? Contact a geotechnical engineer for a soil report, or contact our team on 1800 880 124 — we can guide you to the right specification for your suburb based on the soils we supply into every day.
State permit and engineering requirements
Permit thresholds are height-based starting points only. Proximity to a boundary, the presence of surcharge loads, sloping land, or specific council overlays can trigger engineering and approval requirements at any wall height. Always confirm with your local council before ordering.
New South Wales
Governing framework: Environmental Planning and Assessment Act 1979; State Environmental Planning Policy (Exempt and Complying Development Codes) 2008.
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Under 600mm, setback ≥1m from any boundary, no surcharge: Exempt development — no approval required in most residential zones.
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600mm–1000mm: May qualify as complying development (CDC) or require a development application (DA) depending on the council and zone.
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Over 1000mm: DA or CDC with structural engineering certification required.
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Tiered walls: Two walls within 2m horizontal separation are assessed as a single wall for height purposes.
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Licensed contractor requirement: Walls over 1.0m require a licensed builder (contractor licence, not owner-builder).
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Engineer: Registered Civil/Structural Engineer (CPEng or NPER).
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Approval body: Local council or private certifier (BPB-registered).
Full detail: NSW retaining wall regulations guide.
Victoria
Governing framework: Building Act 1993; Building Regulations 2018; Victorian Building Authority (VBA).
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Under 1000mm, not within 1.5m of a boundary: Generally exempt from a building permit.
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Over 500mm within 1.5m of a boundary: Building permit required.
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Over 1000mm (any location): Building permit required from a registered building surveyor. Structural engineer's certification required.
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Associated with a pool or spa: Permit required regardless of height.
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Heritage or bushfire overlays: Additional requirements — check your council's planning scheme (planning.vic.gov.au).
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Fencing Notice: Required under the Fences Act 1968 before any boundary work — serve on the adjoining owner before commencing.
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Engineer: Registered Civil/Structural Engineer under the Engineers Registration Act 2019.
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Approval body: Private building surveyor or council building surveyor (VBA-registered).
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Timeline: Building permits through private surveyors typically 10–20 business days for straightforward applications.
Full detail: VIC retaining wall regulations guide.
Queensland
Governing framework: Planning Act 2016; Building Act 1975; Queensland Development Code (QDC).
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Under 1000mm, standard residential site: Generally exempt from building approval.
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Over 1000mm: Building approval required. QBCC-licensed contractor required.
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Over 1500mm: Structural engineering certificate from a registered professional engineer (RPEQ) required.
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Brisbane City Council: Has specific local rules — walls over 1.0m near boundaries require neighbour notification. Check Brisbane City Council's online planning portal (cityplan.brisbane.qld.gov.au).
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No statewide threshold: Individual councils under the Planning Act 2016 set their own rules. Sunshine Coast, Gold Coast, Moreton Bay and Logan each have different requirements — confirm with your council before designing.
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Engineer: Registered Professional Engineer Queensland (RPEQ).
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Approval body: Local council or private certifier (QBCC-registered).
Full detail: QLD retaining wall regulations guide.
Western Australia
Governing framework: Building Act 2011; Building Regulations 2012.
- WA has the lowest exemption threshold in Australia.
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Over 500mm within 1.5m of a boundary: Building permit generally required.
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Over 1000mm (any location): Building permit required regardless of boundary proximity. Engineering certification required.
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Local government area variation: City of Perth, City of Stirling, City of Joondalup and City of Swan each have specific local requirements — confirm before lodging.
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Engineer: Registered Civil/Structural Engineer (CPEng or NER).
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Approval body: Local government building surveyor or private building surveyor.
Full detail: WA retaining wall regulations guide.
South Australia
Governing framework: Planning, Development and Infrastructure Act 2016; Planning and Design Code (replaced Development Plan system in 2021).
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Under 1000mm, flat ground, away from boundaries, no surcharge: Generally exempt in most residential zones.
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Over 1000mm: Development approval required from your local council.
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Adjacent to buildings, pools or boundaries: May require approval at lower heights.
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Tiered walls: Assessed on total retained height, not individual wall heights.
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Driveway walls: Driveway surcharge triggers engineering and approval at any height.
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Check: SA Planning Portal (saplanningportal.sa.gov.au) for your property zone and overlays before designing.
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Timeline: Development approval typically 4–8 weeks; fees $600–$1,800 depending on cost of works.
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Engineer: Registered Civil/Structural Engineer (CPEng or NER).
Full detail: SA retaining wall regulations guide.
Tasmania
Governing framework: Building Act 2016; National Construction Code (NCC) adopted as base.
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Over 1000mm: Building permit required. Engineering certification required.
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Approval body: Council building surveyor or private building surveyor.
ACT
Governing framework: Building Act 2004 (ACT).
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Over 1000mm: Building approval required. Engineering certification required.
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Approval body: ACT Planning and Land Authority or private certifier.
Northern Territory
Governing framework: Building Act 1993 (NT).
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Over 1000mm: Building permit typically required.
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Tropical conditions: Corrosion and drainage specification are critical. Confirm post specification and drainage design with a structural engineer for all NT sites.
When engineering is required regardless of height
Even below the permit threshold, a structural engineer should be engaged when any of the following apply:
- The wall is within 1.5m of a property boundary
- A driveway, vehicle parking, structure or significant landscaping sits above the wall
- The site has reactive, expansive or unknown soil conditions
- The wall is on a sloping site where the retained height changes along the run
- The wall is within the zone of influence of an existing building footing
- A swimming pool is adjacent to the wall or within the zone of influence
- The wall forms part of a tiered system where the total retained height across tiers exceeds the individual exemption threshold
Get the engineer's specification before ordering — the engineer may specify a different post series, length or footing than your initial plan.
See the full council approval guide — all states.
Corner, joiner and end posts
Corner, joiner and end posts are available in 45° and 90° return configurations for 100 Series and 150 Series. Use them for:
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End posts: at the termination of every straight wall run — two per run. Use C-channel end posts with the channel facing inward.
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90° corner posts: at right-angle changes in direction — boundary returns, U-shaped planter boxes, recessed garden beds.
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45° corner posts: for angled wall returns — common in driveway batters and diagonal boundary treatments.
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Joiner posts: where two wall runs meet in a T-junction or where a wall run is broken by a gate opening.
Always specify corner and joiner posts in the same series as your standard wall posts. A 150 Series wall needs 150 Series corner posts — mismatched series creates a weak point at the return.
Drainage — what goes behind every post and sleeper wall
No post series compensates for inadequate drainage. Hydrostatic pressure from trapped water is the leading cause of retaining wall failure in Australia across every soil type and every state. Every wall requires:
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100mm slotted ag pipe with geotextile sock at footing level, laid to fall to a legal discharge point
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Geotextile fabric (geo matting) separating the drainage aggregate from retained soil — prevents clay migration and drainage layer siltation
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Minimum 300mm of 20mm free-draining aggregate (clean crushed rock) behind the wall face — 400–500mm in reactive clay or high-rainfall sites
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Weep holes at the base of the wall at 1.2m centres for emergency pressure relief
Never backfill clay directly against the sleepers or within the drainage zone. See the full drainage guide and backfill guide. Browse drainage kits, ag pipe and weep holes.
Get your wall quoted
Once you have your post series, sleeper size and wall dimensions confirmed, request a free quote with delivery to your postcode — or call 1800 880 124 and our team will confirm the correct post specification and provide pricing within one business day.
Trade and bulk pricing available for builders, landscapers and civil contractors. Open a trade account for trade rates, volume pricing and priority dispatch.
Related guides
This guide is for general information only. It does not replace a licensed structural engineer, certifier, geotechnical consultant, or council advice. For walls over 1.0m, walls near boundaries or structures, walls retaining driveways or vehicles, or walls in reactive soils, always confirm specifications with a licensed professional before ordering materials or starting work.
