Post anchors (sometimes called post spikes, post stirrups, or driven anchors) are a popular alternative to setting fence posts directly in concrete. They hold the post above ground level, which reduces rot and makes future replacement far simpler. For most standard residential fencing in New Zealand, installing post anchors is a straightforward DIY job — provided you understand the ground conditions, the type of anchor that suits your situation, and any consent requirements.
Types of Post Anchors Available in NZ
There are several common styles you'll find at stores like Bunnings or Mitre 10:
- Driven spike anchors – A steel spike is hammered directly into the ground, with a socket on top that accepts the post. Best for firm, stone-free soil.
- Bolt-down / plate anchors – A flat base plate is bolted or screwed into an existing concrete slab or hard surface. Ideal for driveways, patios, or decks.
- Screw-in anchors – A large helical screw is wound into the ground using a bar or power tool. Good for softer or sandy soils where driven spikes may not hold well.
- Adjustable stirrup anchors – Similar to bolt-down anchors but with a bracket that allows minor alignment adjustments after fixing.
Choose galvanised or stainless steel anchors for NZ's coastal and high-rainfall environments — bare steel will rust quickly.
Tools and Materials You'll Need
- Post anchors (sized to match your post — typically 90 mm or 100 mm square for standard fencing)
- Fence posts (H4 or H5 treated timber for ground contact, or use H3.2 when the post is held clear of the ground)
- Sledgehammer or post-driver (for spike anchors)
- Driving tool / mandrel (usually supplied or sold separately for spike anchors)
- Spirit level
- String line and pegs
- Tape measure
- Drill and appropriate fixings (for bolt-down anchors)
- Safety glasses and gloves
Step-by-Step: Installing a Driven Spike Anchor
1. Mark Your Post Positions
Run a string line along the fence line and mark each post position with a peg or spray paint. Standard post spacing for residential fencing is typically 1.8 m to 2.4 m, but check your fence panel or rail lengths first.
2. Check for Underground Services
Before driving anything into the ground, contact Dial Before You Dig (0800 248 344 or beforeudig.co.nz) to identify any buried cables, pipes, or drains. Striking a service line is dangerous and costly.
3. Drive the Spike
Slide the driving tool (mandrel) into the socket of the spike anchor to protect it during driving. Using a sledgehammer or post-driver, drive the spike vertically into the ground. Check for plumb (vertical) with a spirit level every few blows — it's much easier to correct early than once it's deep.
Drive the spike until the base plate or collar sits flush with or just above ground level, as per the manufacturer's instructions.
4. Check Alignment
Once all spikes are driven, run your string line again and confirm each anchor is correctly positioned and plumb in both directions.
5. Insert and Fix the Post
Slide the fence post into the socket. Most spike anchors use coach screws or bolts through pre-drilled holes in the socket to clamp the post firmly. Tighten these securely. Check plumb again before fully tightening.
6. Proceed with Your Fence Build
With posts anchored, attach rails and panels as normal.
Installing Bolt-Down Anchors on Concrete
For hard surfaces, bolt-down anchors are fixed using concrete anchor bolts (such as Dynabolts or similar expansion anchors). Drill holes with a hammer drill to the diameter and depth specified by the anchor manufacturer, insert the bolts, position the base plate, and tighten. Allow any cracked or weak concrete to be assessed before loading it with a fence.
Ground Conditions and Limitations
Post anchors work best in firm, compacted soil or clay. They are generally not suitable for:
- Very rocky ground (spikes will deflect or bend)
- Loose, sandy, or waterlogged soil (poor holding strength)
- Slopes steeper than about 10–15 degrees without engineering advice
In poor ground conditions, traditional concrete footings or screw-pile anchors installed by a professional may be more appropriate.
Do You Need a Building Consent?
In New Zealand, many residential boundary fences are exempt from building consent under Schedule 1 of the Building Act 2004, but this depends on the fence height, location, and your local council's rules. As a general guide, fences up to 2.5 m high on a boundary are often exempt — but always confirm with your local council before starting, as rules vary. The MBIE Building Performance website has useful guidance on consent exemptions.
When to Call a Professional
Consider hiring a qualified fencer (you can find local fencers on tradedirectory.co.nz) if:
- Your ground is rocky, very soft, or on a significant slope
- You're building a tall fence (over 1.8–2.0 m) or a retaining fence that holds back soil
- The fence is near a boundary and you're unsure of legal requirements
- You need a fence that meets specific engineering or Healthy Homes standards (for rental properties, see Tenancy Services)
- You're unsure whether a building consent is required
A professional fencer will have the right equipment to drive anchors accurately, assess soil conditions, and ensure the fence is structurally sound.
