How to Set Fence Posts in Concrete — The Right Way for Perth Soil

Your fence posts are the skeleton of the whole job. If they're not straight, not deep enough, or not set properly, the rest of the fence will show it — panels won't sit right, gates won't swing, and the whole thing will look off from the street. Getting posts right is the most important part of a fencing project, and it's worth taking your time here even if you rush everything else.

This guide covers how to set fence posts in concrete for Perth conditions specifically — the sandy coastal soil most of us are working with, the hard rock and compacted ground you'll hit in the hills and midland areas, and the limestone that sits underneath the sand in some of the older coastal suburbs. We'll also walk you through a concreting technique that gives you a stronger, more stable post than just filling the hole to the top.

Perth Soil Types and What They Mean for Your Posts

Perth is not one soil type. What you're digging into depends entirely on where you are, and it changes the way you approach the job.

Sandy coastal soils

If you're anywhere along the coastal strip — Scarborough, Hillarys, Fremantle, Rockingham, most suburbs west of the freeway — you're working in sand. It's easy to dig, which is the good news. The bad news is that dry sand drains fast and doesn't compact well on its own. The main challenge with sand is the hole walls caving in as you dig. A bit of moisture in the sand helps hold it together while you work, but don't flood it — too much water can make the collapse worse. Posts set in sand without proper concreting will lean over time, especially once you've got panels on them catching the wind.

Hard ground — Midland, Guildford, Perth Hills

Head into the Midland and Guildford area, or further into the hills — Kalamunda, Mundaring, Roleystone — and you'll hit hard rock and compacted ground. It's genuinely tough to dig, especially when it's dry. The trick the fencing crews use is water — wet the ground the day before you dig, or pour water into the hole as you go. The water softens the dry compacted soil and makes it much easier to get through with a shovel or auger. If you're hitting solid rock, you'll need a jackhammer. The upside is that hard ground grips posts well once they're in, and you don't need to go as deep as you would in sand.

Limestone beneath the sand

In some of the older coastal suburbs — parts of Cottesloe, Mosman Park, Fremantle, and further south through Rockingham — you'll dig through sand easily enough, and then hit a layer of limestone underneath. You'll know when you reach it because the auger stops and makes a noise you don't want to hear. For a few patches, a rock bar and some persistence will get you through. If the whole run is sitting on limestone at depth, you'll want to hire a jackhammer attachment or a rock auger. Factor that into your planning and your hire costs.

How to tell what you've got

Grab a handful of soil from the hole. If it falls apart when you squeeze it, that's sand. If it's dry, compacted, and hard to break up, that's the kind of ground you'll find in the Midland and hills areas. If you can't get through it at all because it's solid rock, you've likely hit limestone beneath the sand. Different soils need different approaches. If you're digging into hard, dry clay or compacted ground, wetting the area the day before can make a real difference — it softens the soil and makes the auger or shovel go through much easier. For sand, the ground is already easy enough to dig, but the hole walls can cave in as you go deeper. A bit of moisture helps hold the sand together while you work, but don't soak it. Most Perth properties are sand on top, but it's worth knowing what's underneath before you start so you've got the right gear and enough concrete.

How Deep Should Your Post Holes Be?

The general rule is one-third of the total post length should be in the ground. For a standard 1.8 metre fence using a 2.4 metre post, that means your hole needs to be between 600mm and 800mm deep.

• In sand: Go to the deeper end — 750mm to 800mm. Sandy soil doesn't grip the concrete the way harder ground does, so the extra depth gives you more stability. This is especially important if your fence run is exposed to wind, which covers most of Perth.
• In hard ground (Midland, hills): 600mm is usually enough. Hard compacted ground holds the concrete footing well. Going deeper than you need just means more digging and more concrete for no real benefit.
• In limestone: Get to 600mm if you can. Limestone itself is solid, so once you've got concrete bonded to the rock around the post, it's not going anywhere.

Try to make the hole slightly wider at the bottom than at the top — a bell shape. This gives the concrete more grip and makes the footing harder to pull out of the ground. You don't need to go overboard with this, just angle the auger slightly as you go deeper.

What You Need

Get everything together before you start. There's nothing worse than having a post sitting in wet concrete while you drive to Bunnings for a spirit level.

• Shovel, petrol auger, rock bar or jackhammer — a shovel and a bar will get you through a few holes in soft ground. For more than six to eight posts, hire a petrol auger — Bunnings hires them by the four-hour block, or you can go through Kennards or Coates. A one-person auger will handle sand easily. For hard ground or limestone, consider a two-person unit or a rock auger attachment.
• Spirit level — this is the most important tool for the job. You'll be checking plumb on every single post, on two sides, multiple times. Don't skip this.
• Concrete — premixed bags, either quick-set or standard. Plan on one to two 20kg bags per post, depending on your hole size. Buy a couple of extra bags — running short when you're halfway through a run is a headache you don't need.
• Sand — you'll need this for the base of the hole and for packing above the concrete (more on this in the bowl technique section below).
• Water — a bucket and a hose nearby. You'll need water for mixing or activating the concrete.
• Shovel
• Timber braces or prop sticks — to hold posts upright and plumb while the concrete sets. Offcuts of timber and a few clamps or screws will do the job.
• String line — to keep all your posts in a straight line and at a consistent height across the run.

Understanding Your Concrete Options

There are two types of concrete you'll see at Bunnings for setting fence posts, and they work very differently. Picking the right one depends on how much experience you've got — be honest with yourself here, because the wrong choice will cost you.

Standard premix concrete

This is the regular just-add-water premix — the grey bags you've probably seen stacked at Bunnings. It uses Portland cement, which is the traditional concrete chemistry that's been around forever.

How it works: You mix it with water, pour it around the post, and it cures over 24 to 48 hours. During that time it actually shrinks slightly as it dries — the excess water evaporates and the concrete tightens up.

The big advantage: Time. You've got a couple of hours after pouring to check your posts, adjust your line, nudge things into position, and make sure everything is straight before it firms up. That time is valuable when you're learning.

The trade-off: You need to brace every post with timber props while it cures, and you can't start on rails for at least 24 hours (48 in sandy soil). It's a slower job, but you'll get a straight fence.

Rapid set / quick-set concrete

This is a different product entirely — not just "faster concrete." Rapid set uses a different cement chemistry (calcium sulfoaluminate instead of Portland). It sets in about 15 minutes and reaches working strength in an hour or two.

How it works differently: Unlike standard concrete that shrinks as it cures, rapid set actually expands slightly during the early curing phase. The chemical reaction forms crystals that grow and take up more volume, which means the concrete presses outward against the post and the walls of the hole as it sets. That expansion creates a tighter grip — the post gets held more firmly as the concrete cures, not less.

The advantage: Faster, tighter hold, and you can pour the dry mix straight into the hole and add water on top — no wheelbarrow mixing. Experienced fencers love it because they can set a post and move to the next one within 15 minutes.

So which one should you use?

Ask yourself honestly:

• First fence ever? Standard premix. No question. The extra curing time is your safety net.
• Done one or two fences before? Standard premix. You know enough to be dangerous but not enough to work fast under pressure.
• Done dozens of runs and you're confident in your line? Rapid set will save you time and give you a tighter hold. But you need to be quick, accurate, and sure of your string line before you pour.
• Working alone with no one to hold the post? Standard premix with braces. Trying to hold a post plumb, pour concrete, and beat a 15-minute clock by yourself is a recipe for a crooked fence.

If you're reading this guide, standard premix is almost certainly the right choice. Use it, brace your posts, take your time, and you'll end up with a straight fence.

The Concrete Bowl Technique — Step by Step

This is the method that gives you the most stable post with the least amount of concrete. Instead of filling the entire hole with concrete from bottom to top, you're creating a heavy concrete bowl at the base and packing sand above it. It's a better use of materials, and it gives you a solid anchor at the base with good drainage above.

Here's the process, step by step.

1. Dig your hole to depth, plus an extra 50mm. If you're aiming for 750mm deep, dig to 800mm. That extra space at the bottom is for the sand base.
2. Put a layer of sand at the bottom — about 50mm. This gives the post a stable, level surface to sit on. It also lets you adjust the post height slightly by adding or removing sand, which is handy when you're trying to get all your posts to the same height across a run.
3. Place your post in the hole, sitting on the sand. Push it down firmly so it's seated properly.
4. Check it's plumb with your spirit level. Hold the level against two adjacent sides of the post — not just one. A post can be plumb on one face and leaning on the other. Check both.
5. Brace it with timber props. Screw or clamp a length of timber to the post on one side, angled down to the ground. Do the same on an adjacent side. This holds the post plumb while you work and stops it shifting when you pour the concrete.
6. Mix your concrete (for standard premix) or pour the dry mix into the hole (for quick-set, adding water after).
7. Pour concrete around the base of the post, filling to about two-thirds of the hole depth. So if your hole is 750mm deep, you want concrete up to roughly the 500mm mark. The concrete should be heavier and thicker at the bottom, creating a bowl shape. This is where all your structural weight sits.
8. Do not fill all the way to the top with concrete. This is the part most people get wrong. Filling the whole hole with concrete is wasteful and it traps water at the surface instead of letting it drain away.
9. Let the concrete start to firm up. For quick-set, this takes about 10 to 15 minutes. For standard premix, give it 30 minutes to an hour — you want it holding shape but not fully hard.
10. Pack sand around the post above the concrete, filling the rest of the hole. Pour sand in and pack it down firmly around the post. Really compress it — use the end of a piece of timber or the handle of your shovel to tamp it tight. You want the sand packed solid, not just loosely poured in.
11. Check plumb one more time. Make sure the post hasn't shifted during the process. If it has, adjust it now while the sand is still workable.

Over the next couple of days, the concrete cures fully. If you've used rapid set, the slight expansion during curing presses the concrete tighter against the post and the surrounding soil. If you've used standard premix, the weight and bell shape of the footing does the anchoring. Either way, the compacted sand above holds the post firmly in position while letting water drain through instead of pooling on top. The result is a well-drained, stable footing that uses less concrete than filling the whole hole.

Getting Posts Plumb and in Line

This is where patience matters. A post that's a few degrees off plumb will be obvious once the panels go on, and by then it's too late to fix without breaking out the concrete and starting again.

• Plumb means vertical. Check with a spirit level on two adjacent sides of the post. If it reads level on both faces, the post is plumb. If you only check one side, the post could be leaning sideways and you won't know until the panels are on.
• Run a string line along the top of your posts from end to end. Every post in the run should touch the string line at the same point. If one is forward or back, adjust it before the concrete sets.
• Check and recheck. After you pour the concrete, check plumb again. After you pack the sand, check again. Concrete and sand can nudge a post as you work around it, and catching a shift early is easy — catching it after the concrete has cured means starting that post over.
• If a post moves while you're working on the next one, fix it immediately. Don't tell yourself you'll come back to it. Once the concrete firms up, that's where it's staying.

How Long Before You Can Start on Rails?

With standard premix concrete, wait at least 24 hours before attaching rails. In sandy soil, 48 hours is better — sand doesn't support the post the way harder ground does while the concrete is still curing, so give it the extra time.

Don't rush this step. A post that shifts because the concrete wasn't properly set will throw out the whole run. If you're doing this over a weekend, set your posts on Saturday morning and start rails on Sunday — that gives you a full day of curing time and you'll be working with solid posts.

Common Mistakes

These are the ones we see most often, and they're all avoidable.

• Not going deep enough in sand. This is the number one issue in Perth. Sandy soil doesn't grip like harder ground, and a post that's only 500mm deep in sand will start to lean once the fence panels are on and the wind picks up. Go to 750–800mm in sand and you won't have this problem.
• Filling the whole hole with concrete. It feels like the right thing to do — more concrete must mean more strength, right? It doesn't. You're using more material for no real benefit, and you're creating a flat surface at the top that traps water instead of letting it drain. The bowl technique uses less concrete and gives you a more stable, better-draining result.
• Only checking plumb on one side. A post has four faces. A spirit level on the front might read perfectly plumb while the post is leaning sideways. Always check two adjacent sides.
• Moving on before the concrete has set. If you start pulling on posts to attach rails before the concrete has cured, you'll shift them. Quick-set needs at least an hour. Standard premix needs at least 24 hours. Be patient.
• Forgetting to brace. In sand especially, an unbraced post will lean the moment you take your hands off it. Brace every post with timber props before you pour the concrete, and leave the braces on until the concrete has fully set.
• Not using a string line. Eyeballing post alignment across a 15 or 20 metre run never works as well as you think it will. A string line takes two minutes to set up and it's the difference between a fence that looks straight and one that doesn't.

This guide covers general best practice for setting fence posts in Perth soil conditions. Always call Dial Before You Dig (1100) before you start any excavation, and check with your local council if you're unsure about fence height or boundary requirements.