If you've ever wondered where the water goes when it rains, or what happens after you flush the toilet, you're dealing with two very different drain systems. In New Zealand, stormwater and wastewater drains are kept completely separate by law — and for very good reason. Understanding the difference can save you from costly mistakes, council fines, and environmental harm.
What Is a Stormwater Drain?
Stormwater is rainwater and surface runoff that flows off your roof, driveway, paths, and garden. Your property's stormwater system collects this water through:
- Roof gutters and downpipes
- Surface channels and grates
- Soakpits or soak holes (in some areas)
- Piped connections to the council stormwater network or a natural waterway
Because stormwater is relatively clean (though it can carry sediment, fertiliser, and other surface contaminants), it is typically discharged directly into streams, rivers, harbours, or the sea — with little or no treatment. This is exactly why it must never be contaminated with sewage or household wastewater.
What Is a Wastewater Drain?
Wastewater (also called sewage or blackwater/greywater) is everything that goes down your sinks, toilets, showers, baths, dishwashers, and washing machines. This water contains bacteria, nutrients, chemicals, and other contaminants that require treatment before being released into the environment.
Your wastewater drains connect to either:
- The council reticulated sewer network, which carries sewage to a treatment plant, or
- An on-site wastewater system such as a septic tank, where treatment happens on your property
Wastewater pipes are designed to be watertight and are laid to specific gradients so waste flows efficiently without blockages.
Why Must They Stay Separate?
Keeping these two systems separate is a requirement under the NZ Building Code (in particular, clauses G13 – Foul Water and E1 – Surface Water), as well as your local council's bylaws. MBIE Building Performance provides guidance on how these requirements apply to residential buildings.
Cross-connections — where stormwater and wastewater are accidentally or illegally joined — cause serious problems:
- Sewage pollution of waterways, beaches, and harbours
- Sewer overflows during heavy rain (stormwater flooding the sewer system)
- Health risks to the public and environment
- Infringement notices and fines from your local council
- Costly remediation work to fix the problem
BRANZ research has highlighted cross-connection as an ongoing issue in older NZ housing stock, particularly homes built before the 1990s when standards were less strictly enforced.
How to Tell Which Pipe Is Which
On most NZ properties, the pipes are colour-coded or can be identified by their location and what connects to them:
| Feature | Stormwater | Wastewater |
|---|---|---|
| Typical pipe colour | Green or terracotta (older) | Black, grey, or terracotta |
| Connected to | Roof, gutters, grates | Toilets, sinks, showers |
| Discharge point | Stream, kerb, soakpit | Sewer or septic tank |
| Smell | No odour | May have sewage odour |
| Inspection opening | Grated surface gully | Sealed access cover |
Note: Pipe colours are not always consistent, especially in older homes. If you're unsure, always consult a licensed drainlayer.
Common Mistakes Homeowners Make
- Connecting a downpipe to the wastewater drain — this floods the sewer during rain
- Discharging a washing machine or sink to a stormwater gully trap
- Assuming a gully trap on the outside wall is stormwater — it's usually wastewater
- Blocking or concreting over inspection points, making future access impossible
- Installing a new drain without a building consent where one is required
What Can a Homeowner Do Themselves?
Some basic maintenance is fine for homeowners to carry out:
- Clearing leaves and debris from gutters and downpipes
- Unblocking a surface stormwater grate
- Checking and cleaning a soakpit inlet
However, any work that involves connecting, altering, or repairing drain pipes — whether stormwater or wastewater — is restricted work under the Plumbers, Gasfitters and Drainlayers Act 2006. This work must be carried out by, or under the supervision of, a licensed drainlayer. Doing this work yourself without a licence is illegal and can void your home insurance.
When to Call a Professional
Call a licensed drainlayer if you notice any of the following:
- Sewage smell coming from a stormwater grate or outside area
- Slow-draining or gurgling fixtures throughout the house
- Wet patches in the yard that don't dry out after rain
- You're planning a renovation, extension, or new outdoor area that involves drainage
- You've bought an older home and want a drain inspection (CCTV camera inspection) to check for cross-connections
A qualified drainlayer can carry out a CCTV drain inspection, identify cross-connections, and complete any remedial work to bring your property up to code. You can find licensed local drainlayers on tradedirectory.co.nz.