Quick answer: Standard fence posts in Melbourne should be buried 600mm (60cm) deep for fences up to 1.8m high, and 750mm deep for fences 1.8m–2.4m. Coastal areas, sandy soils, and windy sites require deeper installation—often 750mm–900mm—to ensure stability.
Table of Contents
Standard Post Depths by Fence Height
The general rule of thumb in Australia: bury one-third of the post's total length underground. For most residential fences, this translates to:
| Fence Height (Above Ground) | Minimum Post Depth | Recommended Depth |
|---|---|---|
| 1.2m (low fence/front boundary) | 450mm | 600mm |
| 1.5m (standard side fence) | 500mm | 600mm |
| 1.8m (standard rear fence) | 600mm | 750mm |
| 2.0m–2.4m (privacy/tall fence) | 750mm | 900mm |
Important: These depths assume stable clay or loam soil and concrete-set posts. Sandy soil, high-wind areas, or heavy fence panels (e.g., Colorbond with minimal gaps) require deeper installation.
Why One-Third Rule?
This ratio provides adequate leverage resistance. A 1.8m fence experiences wind loads that create rotational force at ground level. By burying 600mm–750mm, the concrete footing acts as an anchor, distributing lateral stress deep into the soil where it's more stable.
Factors That Affect Required Depth
1. Fence Material & Wind Load
- Timber paling (spaced pickets): Wind passes through gaps—standard depth usually sufficient
- Solid Colorbond sheets: Act as a sail in strong wind—add 100mm–150mm to standard depth
- Glass pool fencing: Minimal wind resistance but requires AS 1926.1 compliance—typically 600mm minimum with steel spigots
- Lattice or open styles: Lower wind load—minimum depths acceptable
2. Post Spacing
Posts spaced wider than 2.7m (e.g., 3m spans for rural fences) need deeper footings because they support more fence area. Standard suburban spacing of 2.4m allows standard depth.
3. Slope & Drainage
- Sloped sites: Posts on the downhill side bear more load—dig 50mm–100mm deeper
- Poor drainage areas: Waterlogged soil loses bearing capacity—use deeper concrete footings and consider agricultural drainage
4. Coastal & High-Wind Zones
Properties in Sorrento, Rosebud, Dromana, and coastal Frankston experience salt air and frequent southerly gales. For these areas:
- Add 100mm–150mm to standard depths (750mm minimum for 1.8m fences)
- Use 300mm diameter holes (larger concrete collar)
- Consider steel-reinforced concrete (12mm reo bar down centre of post hole)
Soil Types & Depth Adjustments
Clay Soil (Most of Melbourne)
Characteristics: Dense, sticky when wet, cracks when dry. Excellent bearing capacity.
Post depth: Standard depths (600mm for 1.8m fence) are suitable. Clay expands when wet—ensure drainage around post base to prevent heaving.
Sandy Soil (Coastal Areas)
Characteristics: Loose, free-draining, poor bearing strength.
Post depth: Increase by 100mm–150mm (750mm minimum for 1.8m fence). Bell out the bottom of the hole (wider base than top) to create a larger concrete footing. Consider compacting 100mm of crushed rock at base before setting post.
Rock or Limestone
Characteristics: Extremely hard digging, but excellent holding power once posts are set.
Post depth: Standard depths adequate. Use a rock auger attachment or hire a jackhammer to reach required depth. Alternatively, use surface-mounted spigots with chemical anchors (consult engineer for boundary fences).
Fill or Disturbed Ground
Characteristics: Recently excavated, inconsistent density.
Post depth: Increase depth by 150mm–200mm or dig down to undisturbed natural soil. Consider contacting a licensed fencer for advice—fill can settle over time, causing posts to lean.
Installation Methods: Concrete vs Gravel
Concrete-Set Posts (Recommended for Residential)
Pros:
- Maximum stability and wind resistance
- Prevents timber rot by keeping post above waterline
- Complies with most council and insurance requirements
- Long-term durability (20+ years)
Cons:
- More expensive ($12–$18 per bag rapid-set concrete)
- Permanent—difficult to adjust or remove
- Requires 24-hour cure time before building fence
Best for: Boundary fences, high fences (1.8m+), Colorbond, coastal areas
Gravel-Packed Posts
Method: Pack 20mm blue metal aggregate around post in layers, compacting with a steel rod.
Pros:
- Excellent drainage (prevents timber rot)
- Allows minor post adjustments
- Traditional method for rural fences
- Lower material cost
Cons:
- Less lateral stability than concrete
- Can shift over time in sandy soil
- Not suitable for high-wind or tall fences
Best for: Rural fences, low fences (under 1.5m), areas with high water tables
Hybrid Method (Concrete Collar)
Dig hole 100mm deeper. Add 100mm compacted gravel base (drainage), then pour concrete for top 500mm–600mm. Combines drainage benefits with top-end stability. Popular for timber posts in wet climates.
Step-by-Step Post Installation Guide
Tools & Materials Required
- Digging: Post-hole digger or petrol auger (hire $80–$120/day)
- Measuring: 1200mm spirit level, tape measure, string line
- Setting: Rapid-set post mix (1 bag per post), wheelbarrow, shovel
- Bracing: Timber stakes, 50mm screws, drill
- Safety: Gloves, safety glasses, closed-toe boots
Step 1: Mark Post Positions
- Run a string line along fence boundary (check survey pegs if uncertain)
- Mark post centres every 2.4m–2.7m (standard rail length)
- Spray-mark ground with marking paint
- Call Dial Before You Dig (1100) at least 2 days prior—free service maps underground utilities
Step 2: Dig Post Holes
- Depth: 600mm–900mm (see table above for your fence height)
- Diameter: 300mm (allows 100mm concrete around a 90mm post)
- Pro tip: Hire a one-man petrol auger for $80/day—digs 10 holes in under 2 hours vs full-day manual labour
In Mount Eliza, Mount Martha, and Mornington, you may encounter limestone 400mm–600mm down. If hitting rock, use a rock auger attachment or hire a jackhammer.
Step 3: Add Drainage Base (Optional)
Pour 100mm of 20mm aggregate (blue metal) into base of hole. Compact with a timber ramming rod. This prevents water pooling around post bottom—critical for timber posts to avoid rot.
Step 4: Set First Post
- Insert post: Lower post into hole, centred
- Check height: Measure from ground to post top (should align with string line)
- Check plumb: Use spirit level on two adjacent faces (check at 90° to each other)
- Brace: Screw two timber stakes to post at right angles, staking into ground 1m away
Step 5: Pour Concrete
- Mix rapid-set post concrete per bag instructions (usually 3L water per 20kg bag)
- Pour around post, leaving 50mm below ground level (allows for topsoil/mulch cover)
- Tamp with a steel rod to remove air pockets
- Re-check plumb immediately—you have 10–15 minutes before concrete sets
- Slope concrete surface away from post (sheds water)
Concrete tip: One 20kg bag of rapid-set post mix fills approximately 10L volume. A 300mm diameter × 600mm deep hole = ~42L, so you need 4–5 bags per post.
Step 6: Repeat for All Posts
Work systematically down the fence line. Ensure all post tops align with original string line—this guarantees level rails later.
Step 7: Cure Time
- Rapid-set concrete: Walk-on firm in 30–60 minutes, but leave braces for 4 hours
- Full cure: 24 hours minimum before attaching rails or applying lateral load
- Hot weather: Mist concrete with water after 2 hours to prevent cracking
Step 8: Backfill & Finish
After 24 hours, remove braces. Backfill around concrete collar with topsoil or mulch, sloping away from post. This hides concrete and improves appearance.
Melbourne & Coastal Considerations
Mornington Peninsula & Bayside Suburbs
Properties in Frankston, Seaford, Carrum, and the Peninsula face:
- Salt air: Use hot-dip galvanised or stainless steel fixings (not bright zinc)
- Southerly winds: Increase post depth by 100mm–150mm for fences facing south
- Sandy soil: Bell out hole bases, use larger diameter (350mm) holes
- Coastal regulations: Some councils require engineer certification for fences over 2m in high-wind zones
South-Eastern Suburbs (Clay Soil)
Cranbourne, Carrum Downs, and inland areas have stable clay soil. Standard depths (600mm for 1.8m) are adequate, but:
- Ensure drainage around post base (clay expands when wet)
- Avoid setting posts during wet winter months if possible (clay becomes slippery, difficult to compact)
Heritage & Character Overlays
Suburbs like Hampton and parts of Bayside have heritage restrictions. Check with council—some require:
- Traditional post-and-rail styles (affects post spacing)
- Maximum fence heights (1.2m–1.5m front boundaries)
- Specific materials (timber often mandated over Colorbond)
Common Mistakes to Avoid
1. Insufficient Depth for Fence Height
Problem: Posts lean or fall within 2–5 years, especially after storms.
Solution: Always follow the one-third rule minimum. When in doubt, go 100mm deeper.
2. Narrow Post Holes
Problem: 200mm–250mm holes don't allow enough concrete collar. Post can rotate within hole.
Solution: Minimum 300mm diameter for 90mm–100mm posts. This gives 100mm concrete on all sides.
3. Not Checking Plumb During Pour
Problem: Post sets crooked in concrete—impossible to fix without re-digging.
Solution: Check level on two adjacent faces immediately after pouring, and again 5 minutes later. Concrete begins setting fast.
4. Skipping Dial Before You Dig
Problem: Hit underground electrical, gas, water, or NBN. Repair costs can exceed $10,000, plus personal injury risk.
Solution: Call 1100 or visit 1100.com.au at least 2 business days before digging. Free service, legally required.
5. Using Treated Pine Below Ground Without Proper Rating
Problem: H3-rated pine rots within 5–7 years when buried. Only H4 or H5 is suitable for ground contact.
Solution: Specify H4 CCA-treated pine (green tint) for posts. Better yet, use hardwood (ironbark, spotted gum) for 30+ year lifespan.
6. Ignoring Soil Type
Problem: Standard 600mm depth in sand causes posts to shift and lean.
Solution: Adjust depth based on soil—see Soil Types section above.
Frequently Asked Questions
How deep should a 1.8m fence post be?
For a standard 1.8m high fence in stable clay/loam soil, bury posts 600mm deep minimum. For coastal areas, sandy soil, or high-wind zones, increase to 750mm deep.
Can I use a shorter depth if I use a bigger post?
No. Post diameter affects strength against bending, but depth determines resistance to overturning (leverage). A 150mm post at 450mm depth will still fail faster than a 90mm post at 750mm depth in high wind.
How wide should the post hole be?
Minimum 300mm diameter for 90mm–100mm posts. This allows a 100mm concrete collar on all sides. For 125mm posts or coastal installations, use 350mm diameter holes.
Do I need concrete, or is dirt okay?
For boundary fences and any fence over 1.5m, concrete is strongly recommended. Dirt-packed posts can work for temporary or rural fences under 1.2m, but will shift over time. Gravel-packing is a middle option but inferior to concrete for stability.
How long does concrete need to cure before I build the fence?
Rapid-set post concrete: 24 hours minimum before attaching rails or applying lateral load. Full structural cure takes 7 days, but 24 hours is adequate for residential fences.
What if I hit rock at 400mm depth?
Options:
- Hire a rock auger or jackhammer to reach required depth (recommended)
- Use a shallower depth (450mm–500mm) but increase hole diameter to 400mm and use steel reinforcing (consult fencer)
- Switch to surface-mounted spigots with chemical anchors (requires engineering for boundary fences)
Can I install fence posts in winter?
Yes, but avoid pouring concrete in waterlogged holes. Pump out standing water, add 100mm drainage gravel, then pour. Concrete cures slower in cold weather—allow 36–48 hours in winter vs 24 hours in summer.
Do corner posts need to be deeper?
Yes. Corner and gate posts experience load from two directions. Increase depth by 100mm–150mm (e.g., 750mm for a standard 1.8m fence corner post) and consider using a larger diameter post (125mm vs 90mm).
How do I know if my soil is clay or sand?
Simple test: Dig a small hole 300mm deep. Grab a handful of moist soil:
- Clay: Forms a sticky ball, holds shape, feels smooth
- Sand: Falls apart, gritty texture, won't hold shape
- Loam: Holds shape when moist but crumbles easily—ideal soil
What's the penalty for posts that are too shallow?
Shallow posts lead to:
- Leaning fence within 2–5 years (especially after wind events)
- Post pulling out of ground under lateral load
- Cracked or separated rails due to movement
- Insurance may not cover storm damage if installation doesn't meet standards
- Neighbour disputes if fence falls onto their property
Repair costs ($2,000–$5,000 to re-set posts for a typical 20m fence) far exceed the cost of digging an extra 100mm deeper initially.
Professional Installation Services
Not confident about soil type, depth requirements, or dealing with difficult ground conditions? The South East Fencer provides professional fence installation across Melbourne's south-east and the Mornington Peninsula.
We service:
- Frankston, Seaford, Carrum, Chelsea
- Mornington, Mount Eliza, Mount Martha
- Sorrento, Rosebud, Dromana, Flinders
- Cranbourne, Carrum Downs
Our licensed fencers:
- Assess your soil type and adjust depth accordingly
- Comply with all Victorian council regulations
- Use coastal-grade materials in salt-air zones
- Provide written warranties on workmanship (10+ years)
- Handle all Dial Before You Dig notifications
Final Thoughts
Proper fence post depth is the foundation of a long-lasting fence. While it's tempting to save time or effort by digging shallower holes, the cost of premature failure—both financial and in neighbour relations—far outweighs the extra hour spent digging correctly.
Remember the golden rules:
- One-third underground: Minimum 600mm for 1.8m fences, 750mm for 2m+ or challenging conditions
- 300mm diameter holes: Allows adequate concrete collar
- Adjust for soil: Sandy or coastal locations need deeper installation
- Always concrete-set: For boundary fences and coastal areas
- Dial 1100 before digging: It's free, fast, and can prevent catastrophe
For complex sites, heritage areas, or when you need peace of mind, contact The South East Fencer for expert advice and professional installation throughout Melbourne's south-east and Mornington Peninsula.