There's a number that comes up on almost every fence job we do in Melbourne: 800mm.
It's not arbitrary. It's not a safety regulation someone invented to make work harder. It's what the ground, the climate, and years of watching fences succeed and fail have told us is the minimum for this city.
Melbourne's Soil Moves
Much of Melbourne sits on reactive clay — the kind that swells when wet and shrinks when dry. The moisture content in that soil can shift significantly between seasons, and when it does, it moves whatever is in it. A fence post sitting at 400mm in expansive clay isn't anchored — it's a passenger.
800mm gets you below the most reactive layer on the majority of Melbourne residential sites. In parts of the inner east and the north, you need to go deeper still. But 800mm is the baseline that keeps a fence stable through Melbourne's seasonal extremes.
The Coastal Equation
In the bayside suburbs — Brighton, Sandringham, Black Rock — the soil is sandy, not clay. It's easy to dig and it doesn't swell and shrink the same way. But that doesn't mean you go shallower.
Sandy soil has almost no lateral grip on a fence post. Clay grabs a post and holds it; sandy ground shifts around it. What loose coastal soil gives you in ease of digging, it takes back in holding capacity. We often go to 900mm or more in bayside conditions, depending on fence height and wind exposure.
Height Changes the Equation
Post depth isn't just about soil type — it's about physics.
A fence post is a lever. The top of the fence is the force point. The ground surface is the fulcrum. The buried section is your resistance arm. The taller the fence, the more leverage the wind can apply, and the more depth you need underground to resist it.
Standard practice: total post length should allow for at least one-third underground. For a standard 1.8m fence using a 2.4m post — giving 600mm below ground at minimum — 800mm depth is the correct baseline, with room to go deeper in difficult conditions.
The Concrete Factor
Depth without the right footing doesn't help you. How you fill that depth matters as much as the number itself.
We use properly mixed concrete on every post. Poured wet, without voids, and left to cure before we load any weight onto it. A common failure on budget installs is dry-packed concrete or fill that wasn't given time to set. In Melbourne's wet winters, water infiltration around an inadequately poured footing cracks the concrete over time — and then the post moves.
We pour properly and we wait. 24–48 hours before rails and panels go on. Every time.
What Happens When You Cut Corners
After every major storm season in Melbourne, we see the same thing: fences that went in shallow, in marginal concrete, on posts that weren't set with the site conditions in mind. Five years in, they're leaning.
Corner posts fail first. They take lateral load from two directions. If the footing is shallow or the concrete is weak, the corner goes — and everything connected to it follows. We set corner posts deeper than intermediates as standard. More load, more footing.
The Site Assessment Step
Before we quote a job, we look at what we're digging into. Not just the surface — the profile. In older Melbourne suburbs like Hawthorn, Kew, and Canterbury, you dig through topsoil, hit clay, then hit established root systems that have been growing for decades. In bayside suburbs, you hit sand that changes consistency with moisture.
Each of those conditions changes how the post is set. Fill material can't be relied on for lateral support. Root systems need to be worked around, not through. Old rubble needs to be broken to reach stable ground.
The Difference It Makes
A fence built properly from the ground up will stand for twenty years or more with minimal maintenance. A fence built to minimum spec in difficult conditions starts showing problems in three to five years — leaning posts, movement in the rails, gaps opening at ground level.
The cost difference between a properly set post and a shallow one is measured in minutes and materials. The difference in outcome is measured in years. When we quote a job, 800mm depth isn't an upgrade. It's standard. Because in Melbourne's conditions, anything less isn't building a fence. It's building a problem with a timeline on it.