Backflow Prevention (Boundary)

Backflow is the undesirable reversal of the flow of water from its intended direction. Backflow at a property boundary is dangerous because it can allow contaminants from activities within a property to be drawn into the public drinking water supply.

Boundary backflow can occur as a result of:

• Back-siphon. This is when the pressure drops in the water supply main creating a vacuum in the main and water is sucked from private property into the water supply main. Examples of this include situations where large volumes of water are being drawn off for fire-fighting, when a water main has broken, or when a water main is shutdown for repair work.
• Back-pressure. This is when the water supply pressure from a private premises exceeds the pressure in the water supply main and water is forced from private property into the water supply main. Examples of this include situations where a private water storage tank is installed at a higher level than the water supply network, or when a private pump system is pumping at a pressure which is greater than that of the water supply network.

There are three levels of hazards within a non-residential site:

• High - any condition, device or practice which, in connection with the potable water supply system, has the potential to cause death.
• Medium - any condition, device or practice which, in connection with the potable water supply system, has the potential to injure or endanger health.
• Low - any condition, device or practice which, in connection with the potable water supply system, would constitute a nuisance, by colour, odour or taste, but not injure or endanger health.

A fourth level of Very Low applies to most residences, however there may be activities within a residential property that increase the hazard level.

To protect a potable water supply from the hazards within a property connected to that water supply a backflow prevention device is fitted to the water connection at the point of supply to the property, usually the front boundary.

There are three types of devices used for boundary backflow prevention:

• Reduced Pressure Zone (RPZ) - a testable device used for High level hazards
• Double Check Valve - a testable device used for Medium level hazards
• Dual Check Valve - a non-testable device used for Low level hazards

The boundary backflow prevention device is intended to protect the public drinking water supply from being contaminated from an individual property. It does not protect people working or living within that property from being affected by the contamination, which is the responsibility of the property owner under the Building Act 2004 and subsequent amendments.

Council is required to protect the public supply from the risk of a backflow event as described above. Relevant legislation includes, but is not limited to:

• Health (Drinking Water) Amendment Act 2007
• Building Act
• Local Government Act 2002
• Health Act 1956

Council adopted the Boundary Backflow Prevention for Water Supplies Code of Practice 2006 in 2007, and it was updated in 2013. This document sets out the guidelines for assessing the level of risk from a property and selecting the appropriate level of boundary backflow protection. A copy of the Code can be found on the Water New Zealand website. (See 'Useful Information' below).

All boundary backflow prevention devices will be installed by MDC at the property owners cost.