Backflow is one of those plumbing terms that sounds technical but has a very practical meaning — and ignoring it can put your family's drinking water at risk. In New Zealand, backflow prevention is covered by the Building Code and local council bylaws, so it pays to understand your obligations whether you're building, renovating, or just maintaining your home.
What Is Backflow?
Backflow occurs when water flows in the wrong direction through your pipes — back towards the clean (potable) water supply instead of away from it. This can happen in two ways:
- Back-siphonage: A sudden drop in mains pressure (for example, during a burst main or heavy firefighting demand nearby) creates a siphon effect that sucks water backwards.
- Back-pressure: Pressure in your downstream system (such as a hot water cylinder or irrigation pump) exceeds the incoming mains pressure, pushing water back.
The danger is that contaminated water — from a garden hose submerged in a bucket of fertiliser, a toilet cistern, or a pool filling line — can be drawn back into the drinking water supply for your home or even your neighbours'.
Why It Matters in New Zealand
New Zealand's drinking water is protected under the NZ Building Code clause G12 (Water Supplies), which requires that potable water supplies are protected from contamination. MBIE Building Performance provides the Acceptable Solutions and Verification Methods that spell out how to comply, and local councils can add their own requirements on top of these through water supply bylaws.
BRANZ research has highlighted backflow as an underappreciated risk in residential settings, particularly where hoses, irrigation systems, and appliances connect directly to the mains.
Common Backflow Risks Around the Home
You might be surprised how many everyday connections create a potential backflow hazard:
- Garden hose taps (especially with spray attachments or chemical injectors)
- Irrigation and drip-feed systems connected to the mains
- Swimming pool and spa fill lines
- Washing machine connections
- Dishwashers and under-bench water filters
- Hot water cylinders (thermal expansion can cause back-pressure)
- Rainwater tank top-up connections to the mains
Types of Backflow Prevention Devices
There are several devices used to prevent backflow, each suited to different risk levels:
| Device | How It Works | Typical Use |
|---|---|---|
| Hose-tap vacuum breaker | Atmospheric vent breaks the siphon | Garden taps, low-risk hose connections |
| Double check valve (DCV) | Two spring-loaded check valves in series | Irrigation, appliances, medium risk |
| Reduced pressure zone (RPZ) device | Pressure-differential valve with relief port | High-risk connections, commercial use |
| Air gap | Physical gap between outlet and flood level | Tanks, cisterns — the simplest solution |
For most residential garden taps, a simple hose-tap vacuum breaker (sometimes called a hose-bibb vacuum breaker) is the minimum requirement. These are inexpensive fittings available at Bunnings or Mitre 10, though installation on the water supply side must be done by a licensed plumber.
Do You Need Backflow Prevention?
The short answer for most NZ homeowners: yes, at least at your garden taps. Here's a quick guide:
- Garden/hose taps: A vacuum breaker is required under G12 Acceptable Solutions. If your home was built or renovated in recent decades, you may already have one — check for a small cylindrical fitting on the tap outlet.
- Irrigation systems: Any system with a direct mains connection needs at minimum a double check valve, and possibly an RPZ device depending on what chemicals or fertilisers are used.
- Rainwater tank mains top-up: This is a higher-risk connection and typically requires an RPZ device or a compliant air gap arrangement.
- Swimming pools and spas: Require backflow protection — your pool installer or plumber should have addressed this, but it's worth checking.
- Rental properties: Under the Healthy Homes Standards, landlords must ensure their properties comply with the Building Code, which includes water supply safety.
Maintenance and Testing
Backflow prevention devices aren't fit-and-forget. Testable devices such as DCVs and RPZ valves must be inspected and tested regularly — typically annually — by a certified backflow prevention tester. Your local council may require you to register certain devices and provide test certificates. Check with your council's water services team for local requirements.
Simple hose-tap vacuum breakers should be visually checked periodically and replaced if they show signs of corrosion or damage.
When to Call a Professional
Backflow prevention work on your water supply pipework is restricted plumbing work under the Plumbers, Gasfitters, and Drainlayers Act 2006. This means it must be carried out by — or under the supervision of — a licensed plumber registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB). Doing this work yourself or using an unlicensed person is illegal and could void your home insurance.
You should call a licensed plumber if you need to:
- Install or replace any backflow prevention device on the water supply pipework
- Connect an irrigation system or pool fill line to the mains
- Set up a rainwater tank mains top-up connection
- Have your devices tested and certified (a certified tester, who is usually a licensed plumber, is required)
A qualified local plumber can assess your property, identify any unprotected cross-connections, and install the right devices to keep your water safe and your home compliant. You can find licensed plumbers in your area at tradedirectory.co.nz.