What is backflow?
Backflow is the unintended reversal of water flow or mixtures of water and contaminants into the water supply system.
Why does backflow happen?
Backflow can occur in two ways:
1. Back-siphonage
This happens when water pressure in the public water mains drops suddenly. The pressure difference can create a suction effect, drawing water from private properties back into the public water supply.
2. Backpressure
This occurs when the water pressure on a property becomes higher than the pressure in the public water supply. This can force water and potential contaminants back into the public network.
What is a backflow prevention device?
A backflow prevention device is a mechanical device installed on a property's water connection. It prevents water from flowing in the wrong direction and helps protect the public drinking water supply from contamination.
Property owner responsibilities
Under the Building Act 2004, Selwyn District Council will manage the backflow requirements through the building consent process. However, if you are connected to the public water supply and the use of your property changes, or an appropriate backflow prevention device is not installed, you will be required to install a suitable backflow prevention device.
It's the property owner's responsibility to supply and ensure the appropriate backflow prevention device is in place, and manage its ongoing maintenance.
- All backflow prevention devices must be maintained and fully functional
- All testable backflow prevention devices must be tested annually
Building Warrant of Fitness
If you are the owner of a commercial, industrial or communal (public use) building, Selwyn District Council requires testing of each backflow prevention device annually.
Testing of high and medium risk devices installed under the Building Act 2004 will be carried out annually, or as part of the Building Warrant of Fitness process.
Backflow devices at high and medium risk properties must be tested every year by and testing must be completed by an approved backflow technician or Independently Qualified Person (IQP), at the property owner's expense.
If your backflow prevention device has been installed by Selwyn Water the annual testing will be completed by our network management contractor, CORDE, and you will be charged for this service.
Selwyn Water's role
The Water Services Act 2021 requires drinking water suppliers, including Selwyn Water, to protect the drinking water network from backflow risks.
If Selwyn Water identifies a backflow risk on your property, we may:
- Install a backflow prevention device and recover the costs of installation, maintenance, and ongoing testing from the property owner, or;
- Require the property owner to install, maintain, and regularly test a backflow prevention device that meets Selwyn Water's requirements and includes a verifiable monitoring system.
- If you have a Selwyn Water installed backflow device, you are required to have this tested annually. This is called an annual commercial property backflow audit. Please see our fees and charges schedule for more information.
The type of backflow prevention device required depends on the risk.
Risk levels and device requirements
Categories for backflow prevention are aligned with the degrees of hazard as defined in the New Zealand Building Code, Clause G12 Water Supplies.
Our Backflow Prevention Policy sets out the full requirements for new and existing properties.
The three categories of risk are high risk, medium risk, and low risk. The table below provides examples of the types of risk and the backflow prevention device required, more detail is provided in the New Zealand Building Code.
| Risk | Activity | Examples | Device | Testing |
|---|---|---|---|---|
| High | Uses or produces toxic or bacterial matter that may cause death or serious illness if it entered the water supply | Hospitals, mortuaries, chemical plants, laboratories, cooling towers, hairdressing salons, food processing and other manufacturing plants. | Reduced pressure zone (RPZ) | Annual |
| Medium | Produces backflow that can endanger health | Public and private swimming pools, spa baths, garden irrigation systems (pop-up sprinklers), drink dispensers with carbonators, commercial laundries and rainwater tanks connected to household plumbing. | Testable double-check valve | Annual |
| Low | Could cause a nuisance from colour, smell or taste but does not endanger health | A residential home with only domestic water use | Dual check valve | Not required |
You may need an RPZ device if:
- You use water for washing equipment that has been exposed to chemicals or pathogens
- You mix water with other substances on your property
- You have a secondary water supply on your property
For all enquiries about backflow prevention requirements, please contact us.