Backflow prevention gets over-specified as often as it gets under-specified — both cost someone money, either in unnecessary hardware or in a failed compliance check. The right spec depends entirely on hazard rating, not habit.
Low hazard: dual check valves
A 15mm brass dual check valve, 1200kPa rated (also available in chrome, male x female) is the compliant, non-testable device for low-hazard connections under AS/NZS 3500 — garden taps, washing machine connections, and similar points where the worst-case contamination risk is aesthetic (dirty water), not toxic or health-hazardous. It's not field-testable, which is exactly why it's only compliant for low-hazard applications — there's no requirement to verify it's still working because the consequence of failure is minor.
Medium/high hazard: testable devices
Anywhere the potential contaminant is a health hazard — chemical dosing points, irrigation systems drawing from non-potable sources, medical or lab connections — the applicable standard requires a testable backflow prevention device (RPZ or similar), installed by a licensed backflow tester and verified on a testing schedule. A dual check valve is not a substitute here, regardless of budget pressure — it's the wrong device class for the hazard rating, not a cheaper version of the right one.
Testing your work before it's buried
Whatever device class you've specified, pressure-test the line before it's covered or commissioned. An inflatable test plug with hand pump lets you isolate and test a section without permanent capping — for jobs running multiple pipe sizes, the 50/100/150mm bundle covers the common range without buying one size at a time.
Quick self-check
Ask what the worst realistic outcome is if the device fails. Aesthetic-only (discoloured water, minor contamination) — dual check is likely compliant. Any genuine health risk — you need a testable device and a certified tester, full stop.