Connecting a rainwater tank overflow to a drain sounds straightforward, but in New Zealand it touches on restricted drainlaying work governed by the Plumbers, Gasfitters and Drainlayers Act 2006. Whether you can do it yourself depends on exactly where the overflow discharges — and getting it wrong can result in fines, failed inspections, or drainage problems down the track.
What Is a Rainwater Tank Overflow?
When your rainwater tank fills up during heavy rain, excess water needs somewhere to go. This is the overflow outlet — typically a pipe fitted near the top of the tank. Where that pipe terminates is the critical question:
- To a soakage pit or garden area — generally simpler and may not require a licensed drainlayer, depending on your council's rules.
- To a stormwater drain — this is where restricted drainlaying work almost certainly comes into play.
- To a kerb and channel — requires council approval and usually a licensed drainlayer.
What Counts as Restricted Drainlaying Work?
Under NZ law, drainlaying — which includes connecting pipes to any sanitary drain, stormwater drain, or public sewer — must be carried out by, or under the supervision of, a licensed drainlayer. The Plumbers Gasfitters & Drainlayers Board (PGDB) oversees licensing and can confirm what work requires a licence.
As a homeowner, you are not permitted to connect an overflow pipe directly into a stormwater drain or any council-owned drainage network yourself. Doing so without a licensed drainlayer could:
- Breach the NZ Building Code (particularly Clause E1 – Surface Water).
- Void your home insurance.
- Result in infringement notices from your local council.
- Create liability if the connection causes flooding or damage to neighbouring properties.
What Can a Homeowner Do?
There are parts of the job a homeowner can legally handle:
- Installing the tank itself and its fittings above ground.
- Laying the overflow pipe from the tank outlet across your property — as long as it terminates at a legal discharge point that doesn't connect to a drain.
- Directing overflow to a garden, lawn, or soakage area — provided this complies with your local council's requirements and doesn't cause nuisance to neighbours.
Always check with your local council before starting, as rules vary across regions. Some councils require a building consent for rainwater tank installations, while others may treat it as exempt work under Schedule 1 of the Building Act 2004. Confirm this with your council before assuming consent isn't needed.
Does It Need a Building Consent?
This depends on your council and the specifics of the installation. Under Schedule 1 of the Building Act 2004, some minor drainage work is exempt from building consent — but the exemptions have conditions, and connecting to a stormwater network is generally not exempt. The MBIE Building Performance website has guidance on consent requirements, but your local council has the final say.
Stormwater and the NZ Building Code
Clause E1 (Surface Water) of the NZ Building Code requires that surface water, including rainwater tank overflow, is managed so it doesn't cause damage to buildings or neighbouring properties. Your overflow system must discharge to an appropriate point — and if that point is a stormwater drain, the connection must be properly made and consented.
For guidance on rainwater collection and overflow management, BRANZ publishes technical information relevant to NZ conditions and building practice.
Practical Steps for Homeowners
Here's a sensible approach to getting your overflow connection sorted legally and safely:
- Decide on your discharge point — soakage, garden, or stormwater drain.
- Contact your local council to find out consent requirements and any restrictions on where overflow can discharge.
- If connecting to a stormwater drain, engage a licensed drainlayer to carry out or supervise that part of the work. You can find qualified local drainlayers on tradedirectory.co.nz.
- Check your tank supplier's installation instructions — overflow pipe sizing and fall requirements will be specified.
- Consider a soakage pit as a simpler, often consent-free alternative where soil conditions allow.
Overflow Pipe Sizing — A Quick Guide
| Tank Size | Recommended Overflow Pipe Diameter (minimum) |
|---|---|
| Up to 5,000 L | 50 mm |
| 5,000 – 10,000 L | 65–80 mm |
| Over 10,000 L | 100 mm or as specified by supplier |
Always follow your tank manufacturer's specifications — the above is a general guide only.
Safety Note
Ensure your overflow pipe is fitted with a vermin-proof screen or flap to prevent mosquitoes, rodents, and debris from entering the tank or pipe. This is particularly important for tanks used for drinking water supply.
When to Call a Professional
You should engage a licensed drainlayer whenever your overflow needs to connect to:
- A stormwater drain (private or council).
- A kerb and channel.
- Any underground drainage network.
A licensed drainlayer will ensure the work meets the NZ Building Code, obtain any required consents, and provide you with a Certificate of Compliance — important for your records and future property sales. Find a qualified drainlayer in your area through tradedirectory.co.nz.