Digging post holes in hard clay soil by hand is challenging but absolutely achievable with the right tools, a bit of patience, and good technique. Many New Zealand properties — particularly in Auckland, Waikato, and Hawke's Bay — sit on dense clay that can feel almost concrete-like in dry summer conditions. Here's how to tackle it effectively.
Gather the Right Tools First
Using the wrong tools in clay is the number one reason the job becomes a nightmare. Before you start, make sure you have:
- Clamshell (post hole) digger – the most important hand tool for post holes. The two hinged blades bite into soil and clamp together to lift material out.
- Digging bar (pinch bar) – a heavy steel bar, typically 1.5–1.8 m long, used to break up compacted clay and dislodge rocks before the clamshell digger can work.
- Spade or sharp-edged shovel – useful for cutting the initial outline of the hole.
- Bucket of water – pouring water into the hole softens clay significantly and makes digging much easier.
- Tape measure and string line – to keep your post positions accurate.
- Safety glasses and gloves – essential when using a digging bar.
You can hire or buy clamshell diggers and digging bars from Bunnings or Mitre 10, and many hire centres stock them as well.
Mark Out and Prepare Your Holes
Before you dig, mark each post position clearly with a stake or spray paint. Use a string line to keep your fence line straight. Check with your local council whether your fence requires a building consent — most residential boundary fences under 2.5 m high are exempt under Schedule 1 of the Building Act, but it pays to confirm with your council rather than assume. You can find guidance at MBIE Building Performance.
Also dial 0800 DIG SAFE (0800 344 723) before you dig to check for underground services. This is a free and important step — hitting a buried power or gas line is extremely dangerous.
The Step-by-Step Digging Method
1. Soften the Clay the Night Before
If conditions allow, soak the area with a hose the evening before you plan to dig. Even a few centimetres of moisture penetration makes a real difference to how the clay breaks up.
2. Cut the Outline
Use your spade to cut a clean circle or square slightly larger than your post diameter. For a standard 100 mm fence post, aim for a hole around 200–250 mm wide.
3. Break Up the Clay with the Digging Bar
Drive the digging bar repeatedly into the base and sides of the hole using a firm downward stabbing motion. This fractures the clay into chunks. Work in a circular pattern around the hole rather than just the centre. This is the most physically demanding part — take breaks and stay hydrated.
4. Remove Broken Clay with the Clamshell Digger
Once you've loosened a section, use the clamshell digger to scoop out the broken material. Open the blades, push down, then squeeze the handles together to grip the clay and lift it out. Tip it to the side — not directly next to the hole, as it can fall back in.
5. Add Water as You Go
Every 150–200 mm of depth, pour a litre or two of water into the hole and let it soak in for a few minutes. This is the single most effective trick for hard clay — it dramatically reduces the effort needed with the digging bar.
6. Repeat Until You Reach the Right Depth
As a general guide, posts should be buried to at least one-third of their total length in the ground. For a 1.8 m fence, that typically means a post around 2.4 m long set 600 mm deep. For exposed or coastal sites, or where the fence is taller, deeper is better. Check the requirements of NZS 3604 or consult a fencer for your specific situation.
Dealing with Rocks and Roots
If you hit a rock, the digging bar is your best friend — use it to lever and pry the rock loose. For large boulders, you may need to reposition the hole slightly. Tree roots can be cut with a sharp spade, pruning saw, or reciprocating saw, but be mindful of nearby trees and their stability.
Setting the Post
Once your hole is at the right depth, check it's reasonably plumb using a spirit level. Backfill options include:
- Concrete – the most secure option for gate posts and corner posts. Use a rapid-set mix for convenience.
- Compacted gravel and clay – suitable for intermediate posts in stable ground. Tamp firmly in layers.
- Proprietary post-fix products – expanding foam or dry-pour concrete mixes available at hardware stores.
For timber posts, make sure you're using H4 or H5 treated timber for in-ground use in NZ conditions, as recommended for ground-contact use in NZ conditions. Untreated or under-treated timber will rot quickly in clay, which retains moisture.
When to Call a Professional
If your soil is extremely rocky, you're digging more than 10–15 holes, or the fence is on a boundary where disputes could arise, it's worth getting a qualified fencer involved. A professional fencer will have access to tractor-mounted or petrol-driven post-hole borers that make light work of even the hardest clay. You can find experienced local fencers on tradedirectory.co.nz. Always use a licensed tradesperson for any work near underground electrical or gas lines.
For health and safety guidance when doing physical outdoor work, refer to WorkSafe NZ.
