In most situations, a New Zealand homeowner can dig a pit for an underground rainwater tank on their own property — but before you fire up the excavator or grab a shovel, there are several important steps to work through first. Getting the preparation right will keep you safe, keep you legal, and save you from costly mistakes.
Check Whether You Need a Building Consent
Installing an underground rainwater tank often involves both excavation and plumbing connections, which may require a building consent depending on the scope of work. Under Schedule 1 of the Building Act 2004, some minor work is exempt from consent — but the rules are nuanced and vary by council.
As a general guide:
- A small, freestanding above-ground tank connected only to a garden tap may be exempt.
- An underground tank with plumbing connections to your house is more likely to need consent, particularly if it connects to the potable (drinking) water supply.
- Any work that affects your stormwater or drainage system may also trigger consent requirements.
Always confirm with your local council before you start digging. MBIE Building Performance has useful guidance on consent exemptions, but your council has the final say.
Locate Underground Services First — This Is Critical
Before any excavation, you must identify and mark all underground services on your property. Striking a buried power cable, gas line, or water main can be fatal or cause serious damage.
- Contact Dial Before You Dig (0800 248 344) or visit their website to request a service location plan.
- Your local lines company, gas network operator, and council can also provide records.
- Even with plans, consider hiring a cable and pipe locator for a physical sweep — plans are not always accurate.
WorkSafe NZ has clear guidance on excavation safety, including the risks of unlocated services.
Understand the Soil and Site Conditions
The type of soil on your site will determine how stable the pit walls are and whether they need shoring or battering (sloping the sides) to prevent collapse.
| Soil Type | Risk Level | Typical Action Required |
|---|---|---|
| Firm clay | Lower | May stand unsupported short-term |
| Sandy or loose soil | High | Shoring or battering essential |
| Waterlogged ground | Very high | Specialist advice needed |
| Fill or disturbed ground | High | Shoring or battering essential |
Under WorkSafe NZ guidelines, any excavation deeper than 1.5 metres must be shored, battered, or otherwise made safe before anyone enters it. Even shallower excavations can collapse without warning — never enter an unshored pit.
What Digging Work Can a Homeowner Do?
The excavation itself — physically digging the hole — is generally not restricted work in New Zealand. A competent homeowner can:
- Hand-dig or hire a small excavator (many hire companies offer mini-diggers for DIY use)
- Prepare the base with compacted sand or gravel bedding as specified by the tank manufacturer
- Backfill around the tank once it is installed
However, the plumbing connections are a different matter entirely.
Plumbing and Drainage — Restricted Work
Connecting your rainwater tank to any household plumbing is regulated work under the Plumbers, Gasfitters, and Drainlayers Act 2006. This includes:
- Connecting the tank to your roof downpipes (if involving new pipework inside the boundary)
- Any connection to the potable water supply (which also requires backflow prevention devices)
- Overflow connections to the stormwater or drainage system
This work must be carried out by a licensed plumber or drainlayer registered with the Plumbers Gasfitters & Drainlayers Board. Doing this work yourself is illegal and could void your home insurance.
If your tank system includes a pump with electrical connections, that wiring must be done by a registered electrician — see the Electrical Workers Registration Board for more information.
Tank Placement and Structural Considerations
- Follow the tank manufacturer's installation instructions precisely — incorrect bedding or backfill can crack or deform the tank.
- Keep the pit well clear of building foundations, retaining walls, and boundary fences. A general rule of thumb is to maintain at least the depth of the excavation as a setback from any structure, but check with your council or a structural engineer if in doubt.
- In areas with high groundwater, an empty tank can float out of the ground — your installer or engineer can advise on anchoring.
Rainwater Use and the NZ Building Code
If you intend to use harvested rainwater for drinking or household use, the system must comply with MBIE Building Performance requirements under the NZ Building Code (particularly Clause G12 — Water Supplies). BRANZ also publishes useful guidance on rainwater collection systems for New Zealand conditions.
For general information on water efficiency and rainwater use, Energywise is a helpful resource.
When to Call a Professional
Consider hiring a licensed earthmover or excavation contractor if:
- The pit is deeper than 1.5 metres
- You have unstable, sandy, or waterlogged soil
- The site is near existing structures, retaining walls, or trees with large root systems
- You are unsure about underground services
- You need the work done quickly and safely with the right machinery
For all plumbing and electrical connections, a licensed tradesperson is not just recommended — it is legally required. You can find qualified local earthmovers, plumbers, and electricians on tradedirectory.co.nz.
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