What is Backflow

Backflow is the unplanned reversal of flow of water (or water and contaminants) into the public water supply system.

Whenever water is supplied to a property from the public water supply system, the intention is for the water to flow from the public supply system to that property. However, the flow of water could be reversed from the property back into the public supply system. If cross connections exist within the customer’s plumbing system when backflow occurs, then it is possible to contaminate the public water supply.

A common situation is the end of a garden hose submerged in a bucket or other container of liquid. Backflow can also arise from appliances appliances and pools.

There are 3 common types of Backflow; back siphonage, back pressure and cross connections.

IQP Testing and Backflow Devices

Water quality and contamination has become an increasing concern for both councils and building owners throughout New Zealand. A backflow device will ensure water can only pass through a pipeline in one direction. There are many devices available and the one that is right for you will depend on the water supply system you have, the activity being carried out and the risk of contamination.

As part of a Buildings WOF it is a requirement that testable Backflow prevention devices are tested annually by an Independently Qualified Person (IQP).

Canterbury Backflow Prevention have IQP registered staff that can test and service your device. Valves will be tagged and a 12A issued to assist in the completion of the Building WOF with the Council.

Examples of High Risk

  • Agricultural (Dairy Farms)
  • Bottling plants
  • Car washes
  • Chemical plants
  • Food processing facilities
  • Hospitals
  • Laboratories
  • Petroleum processing or storage plants
  • Wastewater treatment plants

Examples of Low Risk

  • Boilers
  • Hose bibs (inside and outside garden hose faucets)
  • Lawn irrigation systems
  • Solar heat systems
  • Swimming pools
  • Wash basins

Back Siphonage

Backsiphonage occurs when there is a partial vacuum (negative pressure) in a water supply system, which draws the water from a contaminated source into a potable water supply. The water pressure within the public supply system falls below that of the plumbing system it is supplying. The effect is similar to siphoning or drinking water through a straw. For example, during a large fire, a pump is connected to a hydrant. High flows pumped out of the distribution system can result in significantly reduced water pressure around the withdrawal point. A partial vacuum has been created in the system, causing suction of contaminated water into the potable water system. During such conditions, it is possible for water to be withdrawn from non-potable sources located near the fire — for example, air-conditioning systems, water tanks, boilers, fertilizer tanks and washing machines — into buildings located near a fire. The same conditions can be caused by a water main break.


Back-pressure occurs when the pressure of the non-potable system exceeds the positive pressure in the water mains supply system. For example, there is a potable water connection to a hot water boiler system that is not protected by an approved backflow preventer. If pressure in the boiler system increases to a point that it exceeds the pressure in the water mains supply system, a backflow from the boiler to the public water supply system may occur.

Cross Connection

Backflow can only occur where there is a connection or cross-connection. Cross-connections can occur in any situation where fixtures are connected directly to the main supply and a source of contamination. For example, a garden hose left in a swimming pool. The hose does not necessarily have to be in contact with a hazard to require a backflow preventative to be installed. If there is potential for the hose to come in contact with a hazard, a backflow preventative must be installed.

Other common cross connections are:

  • irrigation systems
  • dishwashers,washing machines
  • coffee machines
  • swimming pools, spa pools or ornamental pools that are filled by hose
  • pesticide and fertiliser attachments for hoses
  • fridges and icemakers
  • retractable spray outlets to tubs and sink
  • flexible shower hoses
  • storage tanks.