Boundary backflow prevention – keeping our drinking water safe

We supply good quality drinking water to around 49,000 homes and businesses. We manage the city’s catchments, treat raw water and provide a piped network to get water to your taps.

To keep our drinking water safe, we need your help to prevent contamination from backflow.

What is backflow?

Backflow happens when water (possibly containing contaminants) flows from a property back into the network. Water pressure within the network ensures water is delivered to your property. However, if a property’s water pressure is higher than the network’s, or the water pressure changes in the network, backflow can occur.

Pressure differences between the network and a property can occur when:

• heavy water use downstream reduces water pressure upstream e.g. fire fighting
• a water main breaks or is shut off
• a customer uses water at a higher pressure than the pressure supplied
• the water outlet at the property is higher than the water main.

What are the risks of backflow occurring?

Any activity you undertake that makes a connection with your plumbing poses a risk. At home, filling a bucket, watering can, swimming pool or spa by submerging the hose, leaving a garden hose running in a compost bin, submerging spray heads from showers or sinks, or using chemical cleaning products designed to connect to your hose can all pose a risk.

Chemical or microbiological substances from commercial activities can endanger public health if they get into the water system. Examples of commercial operations that can create a risk include medical and dental facilities and mortuaries, car and factory washing facilities, manufacturing, commercial laundries, food preparation facilities, and fire sprinkler and irrigation systems.

The potential to cause harm depends on the nature of the activity and the likelihood of backflow occurring. For example, an office may be considered low risk, whereas a wastewater treatment plant is considered high risk. There are many examples of backflow incidents around the world where substances including chemicals, cleaning solvents, wine, milk, dirty irrigation water and soapy water have flowed back into the public water network and caused sickness or death.

How is the water supply protected from backflow?

Backflow can be avoided by using backflow prevention devices: mechanical devices that create a physical barrier to backflow. A backflow prevention device is installed at the boundary of a property (often referred to as a “boundary” or “containment” device) to prevent backflow from a property into the network.

Backflow within a building may create a health risk to building users. Where this is the case, a backflow device (often referred to as an “internal”, “zone” or “individual” backflow device) is installed at an appropriate location within the building. It is the responsibility of the building owner to protect building users under the Building Act 2004. You can find out more about internal backflows in the related information section.

Types of boundary backflow prevention devices include:

• reduced pressure zone (RPZ)
• double check valve (DCV)
• air gap.

We will decide which device is required based on the potential hazard from a property. An RPZ provides the highest level of protection. Water tanks on rural water supply schemes must have an air gap. RPZ and DCV backflows require periodic maintenance and annual testing.

For domestic connections made since 1991, we install a manifold at the boundary which contains a valve designed to prevent backflow.

What are the DCC’s responsibilities?

As the water supplier, under the Water Services Act 2021 and the Drinking Water Quality Assurance Rules 2022, we are required to manage backflow at the property boundary to protect the water supply network.

The DCC approves all devices and their installation, and we may ask you to upgrade any non-compliant devices.

We are working through a programme to make sure test able backflow devices are installed on properties that present a backflow risk. We also test each device at least once a year to make sure it is working correctly.

For older domestic connections, we are gradually upgrading service valves to manifolds.

What are my responsibilities?

It is your responsibility to prevent backflow. If you are a non-domestic customer without a testable backflow device, please contact us about installing one. You must also maintain your device in good working condition. You must install a device once we have asked you to do so.

You must make sure there are no cross-connections between the DCC supply and any other water source or substance on your property. A cross-connection is a physical connection from your pipes or fixtures to anything else, e.g. leaving a hose submerged in a bucket, sink or pool.

How do I get a new device installed?

Testable backflow devices are owned by customers and located on private property. You will need to engage a plumber to arrange installation.

Installing a new device also requires a building consent or an exemption from the Building Act. Before installing a device, you or your plumber must submit your proposal to us for approval.

For more information on our installation requirements, seethe Dunedin City Council Requirements for Installing Boundary Backflow Prevention Devices brochure in the related information section.

All costs associated with installing the device are the customer’s responsibility.

Building Act exemptions

An exemption from the Building Act may be given where the backflow installation is the only work that requires a building consent and the work meets our requirements.

There is no cost for processing an exemption application, but the application must be submitted to us before the device is installed. The application form is available in the related information section above.

Building consent

Where there is associated work requiring building consent, the new device will be approved through the building consent process.

Inspection and sign-off

Once the new device is installed, a new device notification form must be submitted to initiate our final inspection, approval and testing. The new device notification form is available in the related information section above.

Annual testing

We test your boundary backflow prevention device at least once a year. There is a charge for the annual test.

We aim to give you at least a couple of weeks’ notice before the testing. Please let us know if there are any access issues, such as padlocks on the device or the property, or other security measures.

Unless a bypass has been installed, the water supply to the property will need to be turned off during the test, which generally takes around 30 minutes. We will let those onsite know before this happens.

When the device passes its test, you will be provided a copy of the test certificate.

If your device fails, we will notify you to fix the device. If it is not fixed within the agreed time frame, a restrictor may befitted.

In specific cases, we may agree to you managing your own testing through an independently qualified person.

Further queries?

If you would like to know more about boundary backflow protection, please call 03 477 4000, or email backflow@dcc.govt.nz.

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