A fence that leans, wobbles, or blows over in a Northerly is almost always the result of posts that weren't set deep enough. Getting the depth right from the start saves you a costly re-do and keeps your fence looking sharp for years to come. Here's a practical guide to setting fence posts in concrete the right way, New Zealand conditions and all.
The General Rule of Thumb
The most widely used guideline in NZ fencing is to bury one-third of the total post length in the ground. So if your fence is 1.8 m high and you're using a 2.4 m post, roughly 600–800 mm should be below ground level. As a minimum, most experienced fencers recommend at least 600 mm for a standard residential fence, and deeper for taller or more exposed fences.
This rule accounts for the leverage forces a fence panel creates — the taller the fence, the more wind load it catches, and the deeper the post needs to go to resist tipping.
Recommended Depths by Fence Height
Use this as a starting guide. Always adjust for your specific site conditions (see below).
| Fence Height | Minimum Post Depth | Typical Post Length |
|---|---|---|
| Up to 1.2 m | 450–600 mm | 1.8 m |
| 1.5–1.8 m | 600–750 mm | 2.4 m |
| 1.8–2.1 m | 750–900 mm | 2.7–3.0 m |
| Over 2.1 m | 900 mm+ | 3.0 m+ |
These are general guides only — your fencer may specify deeper holes depending on the situation.
Why NZ Conditions Matter
New Zealand's varied climate and geology mean a one-size-fits-all approach doesn't always cut it.
- Wind zones: Coastal and exposed sites (think Wellington, Manawatū, or Canterbury plains) experience significantly higher wind loads. Posts in these areas should be set deeper and may need larger diameter holes with more concrete.
- Soil type: Sandy, loose, or waterlogged soils offer less lateral support than firm clay or compacted fill. In soft soils, go deeper and use a wider concrete footing.
- Frost: Parts of the South Island and Central Plateau experience ground frost. Concrete footings should extend below the frost line to prevent heaving.
- Slope: On sloping ground, the downhill post takes more load. Consider going deeper on the downhill side.
BRANZ publishes guidance on wind zones and building exposure categories that can help you assess your site.
Choosing the Right Post Material
Timber posts are the most common choice in NZ. Use H4 or H5 treated timber for in-ground use — H4 for most residential applications, H5 for ground contact in high-decay-risk areas or where posts will be in contact with concrete and soil. Never use untreated or H3 timber in the ground; it will rot quickly.
Steel posts (galvanised or powder-coated) are increasingly popular, especially for modern-style fences. They're durable but must be properly sealed at the base to prevent corrosion where they meet concrete.
Concrete posts are very durable and low-maintenance, though heavier to work with.
How to Set a Post in Concrete: Step by Step
1. Dig the hole
Use a post-hole borer (manual or machine-hire) to dig a clean, straight hole. The hole diameter should be roughly 2–3 times the post width — typically 200–300 mm for a standard timber post. A clean-sided hole gives the concrete something to grip.
2. Add a gravel base
Pour 100–150 mm of coarse gravel or crushed rock into the bottom of the hole. This improves drainage and reduces the chance of the post base sitting in water, which accelerates rot and corrosion.
3. Set and brace the post
Place the post, check it's plumb (vertical) with a spirit level, and brace it securely with temporary timber braces before pouring. Once concrete goes in, you won't be able to adjust easily.
4. Mix and pour concrete
Use a bagged rapid-set or standard concrete mix — products are available at Bunnings or Mitre 10. For a standard 200 mm diameter hole at 600 mm depth, you'll typically use one to two 20 kg bags per post, but calculate based on your actual hole volume. Mix to a firm (not sloppy) consistency and pour in layers, rodding out air pockets.
5. Crown and cure
Mound the concrete slightly above ground level and slope it away from the post — this sheds water and prevents pooling at the base. Leave braces in place until the concrete has fully cured (follow the product instructions; rapid-set mixes can be ready in a few hours, standard mixes need longer).
Common Mistakes to Avoid
- Too shallow: The most common error. Shallow posts will lean over time, especially in wind.
- Flat or concave concrete top: Water pools around the post and accelerates decay.
- Skipping the gravel base: Poor drainage rots timber posts prematurely.
- Not checking for plumb: A post that's even slightly out of vertical looks worse as the fence goes up.
- Using the wrong timber treatment class: H3 timber is not rated for in-ground use.
When to Call a Professional
While setting fence posts is a manageable DIY job for many homeowners, there are situations where it's worth bringing in a qualified fencer:
- Retaining fences or walls over 1.5 m may require a building consent under the NZ Building Code — check with your local council.
- Boundary fences involve legal obligations under the Fencing Act 1978 — a professional can help you navigate neighbour agreements.
- Rocky or difficult ground may need machine boring or specialist equipment.
- Large or commercial projects where structural integrity is critical.
You can find qualified, local fencers on tradedirectory.co.nz — it's always worth getting a couple of quotes and checking their experience with your fence type.