For most NZ fence posts, a 1:2:3 mix (one part cement, two parts sand, three parts aggregate) gives a solid, durable footing. If you'd rather skip the measuring, a bagged rapid-set concrete product is a reliable and convenient alternative — just add water and pour. Either way, getting the mix and post depth right is the key to a fence that stays upright through our NZ winds and wet winters.
The Standard Mix Ratio Explained
The 1:2:3 ratio is a general-purpose concrete mix widely used in NZ trade practice for fence post footings. Here's what each component does:
- 1 part cement — the binder that hardens everything together
- 2 parts sharp sand — fills gaps and adds workability
- 3 parts 20 mm aggregate (crushed stone or gravel) — provides bulk strength
Add water gradually until the mix is firm but workable — roughly the consistency of peanut butter. Too much water weakens the finished concrete significantly, so resist the urge to make it sloppy.
For a small number of posts, a bagged rapid-set or post-fix concrete (available at Bunnings or Mitre 10) is a practical shortcut. These products are pre-blended and some can even be poured dry into the hole and watered in place — check the manufacturer's instructions carefully.
How Much Concrete Do You Need?
As a rough guide, a standard 100 mm round timber post in a 200 mm diameter hole will use approximately one to two 20 kg bags of bagged concrete per post, depending on hole depth. For larger posts (e.g. 125 mm or 150 mm square), you may need three or more bags.
Use this simple approach to estimate:
| Post size | Hole diameter | Hole depth | Approx. bags (20 kg) |
|---|---|---|---|
| 75–100 mm round | 200 mm | 600 mm | 1–2 bags |
| 100 mm square | 250 mm | 700 mm | 2–3 bags |
| 125–150 mm square | 300 mm | 800–900 mm | 3–4 bags |
These are approximate figures only — always check your specific product's coverage rate on the bag.
Post Depth: Just as Important as the Mix
Even the best concrete won't save a post that isn't deep enough. A common NZ rule of thumb is to bury one-third of the total post length in the ground. For a 1.8 m fence, that means a post at least 2.4 m long with roughly 600–800 mm in the ground.
In high-wind zones (common in Wellington, coastal areas, and exposed rural sites), deeper footings and larger diameter holes are advisable. Check with your local council or a qualified fencer if you're unsure — wind zone requirements are referenced in NZS 3604 for timber-framed construction, and similar principles apply to fencing.
NZ Soil and Climate Considerations
- Clay soils expand and contract with moisture, which can push posts over time. A wider, deeper footing helps anchor posts against this movement.
- Sandy or loose soils drain well but offer less grip — again, go deeper and wider.
- Wet regions (Westland, Northland, parts of Waikato) benefit from using a concrete that sets quickly to reduce the risk of the mix being diluted by groundwater.
- Treated timber posts rated H4 or H5 are required for in-ground contact in NZ — check the BRANZ guidelines for timber treatment levels.
Mixing Tips for a Strong Result
- Keep the post plumb — use a spirit level and temporary bracing while the concrete sets.
- Crown the concrete slightly above ground level and slope it away from the post to shed water. This reduces rot at the critical ground-line zone.
- Don't disturb the post for at least 24–48 hours after pouring, or as directed on bagged product instructions.
- Avoid mixing in very cold conditions (below about 5°C) — concrete cures poorly in the cold. If you must work in winter, use a rapid-set product and protect the footing from frost overnight.
- Use clean water — never use seawater or water with high silt content.
Do You Need a Building Consent?
In most cases, standard residential fences under 2.5 m high are exempt from building consent under Schedule 1 of the Building Act 2004, but there are exceptions — for example, fences near boundaries, retaining walls, or swimming pools. Always confirm with your local council before starting, as rules vary by region and property type. More information is available at MBIE Building Performance.
When to Call a Professional
While mixing concrete for fence posts is well within DIY reach, there are situations where it pays to bring in a qualified fencer:
- Long fence runs with many posts — a professional will have a post-hole borer and the experience to work efficiently
- Rocky or very hard ground — specialist equipment is needed
- Retaining fences or walls — these may require engineering input and building consent
- Pool fencing — must comply with the NZ Building Code clause F9 and local council requirements; non-compliant pool fencing is a safety and legal risk
- Sloping or unstable ground — post placement and footing design becomes more complex
You can find qualified, local fencing professionals on tradedirectory.co.nz to get the job done right.
