Hazard class (backflow)

The hazard class is the risk rating of a water connection, low, medium, or high hazard, under AS/NZS 3500.1, that determines what backflow-prevention device is required, from a simple check valve up to a testable reduced-pressure-zone (RPZ) valve.

Backflow is water flowing the wrong way, from an installation back into the drinking-water supply, which can draw contamination into the mains. The hazard class rates how bad that contamination would be if it happened:

• Low hazard: a nuisance, not a health risk (for example, a connection where the worst case is discolouration). A basic device such as a dual check valve suits it.
• Medium hazard: could make someone sick but not cause death (many commercial and irrigation connections). A testable device, often a double check valve, is required.
• High hazard: could cause serious illness or death (chemical dosing, recycled water, certain industrial or medical connections). A reduced-pressure-zone (RPZ) valve, the highest-protection testable device, is required.

Higher hazard means a more capable, and usually testable, device, and testable devices must be commissioned and tested periodically by an accredited person.

For a builder the practical point is that the hazard class drives both the device selection and an ongoing testing obligation, so it is not just a fit-and-forget item. Get the hydraulic consultant or licensed plumber to assess the hazard class for each connection (rainwater-to-mains change-over, irrigation, fire services, dental or lab fit-outs are common triggers), specify the matching device, and register testable devices with the water authority so the annual testing happens.

Also known as: Backflow hazard rating, cross-connection hazard class.

Category: Plumbing / Backflow.

Related

• Backflow prevention

See also

• NCC ventilation and drainage

References

• AS/NZS 3500.1 Water services, Standards Australia (verified 2026-06-01)

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Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.