Preventing irrigation backflow

When using water together with potentially hazardous substances, there is the potential that a loss of pressure could lead to backflow, causing contaminants to enter the water source. 

This is especially important when the water source is used for drinking water or connected to a sensitive environment.

Fertigation and effluent injection systems that use irrigators connected directly to a water source (for example, groundwater abstracted from a bore, or water taken straight from a river) need a functioning system that prevents backflow from occurring in the case of pressure loss.

How to prevent backflow

If you are using a hazardous substance (like fertiliser) in an irrigation system, you will need to install a device that stops backflow entering the water source in the case of a loss of pressure. 

The type of backflow preventer you need will depend on where you source your water, the risk associated with your irrigation practice, and land and water use in the environment.

Irrigators that are connected to the same water source used for drinking water, or in a sensitive environment are not necessarily at a higher risk of backflow occurring, but the consequences of backflow would be severe, so are considered higher risk than other systems.

The use of a chemigation valve is appropriate where the risk of water contamination is low. If the risk of contamination is high, an air gap or RPZ device must be installed.

An air gap system or RPZ will also be required if the water source supplies water for community use or is located close to a community water supply.

Determining what type of backflow prevention system to use is crucial to ensuring you are compliant.

Types of backflow prevention

Air gap

An air gap backflow prevention device is simply a break in plumbing on a vertical pipe, where gravity keeps water flowing under normal operation. In case of a loss of pressure, the air gap naturally prevents backflow from occurring.

Air gaps are commonly used in household plumbing and can also be appropriate for use in on farm fertigation, however, as the air gap leads to a loss of in-pipe pressure, it may require the installation of an additional pump.

Members of the master plumbers, gasfitters and drainlayers association can test your air gap device. 

Chemigation valves

Chemigation valves are often the most cost-effective type of backflow preventers. 

The use of a chemigation valve is appropriate where the risk of water contamination is low. This will apply if there is only a slight chance of backflow reaching the water source in case of a loss of pressure, and if the consequences of such backflow would be minor. 

Chemigation valves need to be tested at the time of installation and annually thereafter by a suitably qualified or certified person. This test certificate must be provided to us.

If you are considering installing a chemigation valve, please read Requirement for the use of chemigation valves for backflow prevention in Canterbury (PDF, 533KB)

Reduced pressure zone device (RPZ)

An RPZ device provides a higher standard of backflow prevention, necessary in a high risk environment. An RPZ consists of two check valves on either side of a differential pressure relief valve.

RPZs must be tested at the time of installation and annually thereafter by a suitably qualified or certified person, and the test certificate provided to us.

Contact Water NZ or Master Plumbers NZ to find a suitably qualified person to field test your RPZ.

Installing a backflow prevention device

If you are looking to install a backflow prevention device, please send us a completed Chemigation/Wastewater Injection Approval Application form.

You will need to be ready to provide us with the following:

  • The consent holder's name, property address and appropriate consent number
  • Details of the water source
  • Details of the installation
  • The type of backflow prevention device, and details
  • Contact details of the installer.

We will let you know if we need any information, as well as whether your installation plan is approved.

Your backflow prevention device should be inspected and tested at the time of installation and annually thereafter by a suitably qualified person.

Test reports should be provided to Environment Canterbury as an attachment to an email to with the subject line: Attention: Regional Manager, RMA Monitoring and Compliance. This should take place within two weeks of testing.

What if I don’t have a backflow prevention device installed?

Where appropriate backflow prevention equipment or infrastructure is not available to prevent backflow of contaminants from directly entering the water source, resource consent may be required.

Rules 5.98 and 5.100 of the Canterbury Land and Water Regional Plan apply to the discharge of water or contaminants onto or into land in circumstances where a contaminant may enter groundwater. In this case the discharge activity is onto land and is associated with the injection of liquid nitrogen (contaminants) into the irrigators directly is connected to groundwater via the bore.

An application for resource consent under Rule 5.100 is required if you cannot meet the relevant conditions of Rule 5.98, one of which is that discharge does not, in groundwater, render fresh water unsuitable or unpalatable for consumption by animals or humans; and the discharge does not contain any hazardous substance.

Need help?

If you have any questions about backflow prevention, contact your farm advisor, Land Management Advisor or call Customer Services on 0800 324 636.