SNZ Plumbing Estimating · 2026-06-02

Backflow Prevention: When RPZD Is Required & Tundish Setup

PCA Part B5 explained – RPZD for high-hazard cross-connections + correct tundish discharge (trap vs in-wall) for safe, compliant installations

Backflow prevention is one of the most critical parts of any plumbing system in Australia. A single unprotected cross-connection can contaminate the entire drinking water supply — which is why the Plumbing Code of Australia (PCA) Part B5 makes backflow prevention mandatory for high-hazard situations.

The highest level of protection is the Reduced Pressure Zone Device (RPZD). Here’s exactly when you need it and how to set up the required tundish discharge.

When RPZD Is Required (PCA Part B5 & AS/NZS 3500.1 Clause 8)

PCA BP5.1 (Performance Requirement): The drinking water supply must be protected against contamination from backflow.

Deemed-to-Satisfy solution in AS/NZS 3500.1:2025 Clause 8.2 and Table 8.1 (Backflow prevention device selection):

• RPZD is required for high-hazard cross-connections — situations where backflow could introduce toxic substances, sewage, or chemicals into the potable system.
• Common high-hazard examples where RPZD is mandatory:
  • Connection to rainwater tanks (if used for non-drinking purposes like toilets/laundry)
  • Irrigation systems with fertiliser injection or chemical dosing
  • Fire sprinkler systems with chemicals or stagnant water
  • Commercial kitchens (dishwashers, grease traps, boilers)
  • Laboratories, mortuaries, dental clinics
  • Car washes with detergents/chemicals
  • Cooling towers or industrial processes
  • Any connection to a non-potable source (e.g., recycled water, bore water)

• Low-hazard situations (e.g., garden taps without chemicals) only need a dual-check valve or non-return valve.
• RPZD location: Installed at the boundary or point of hazard, accessible for annual testing (mandatory in NSW/VIC/QLD for high-hazard devices).

Tundish Setup for RPZD Relief Discharge

RPZD relief valves can discharge intermittently (pressure drop or testing) — this water must be safely directed.

Correct setup (AS/NZS 3500.1 Clause 8.5 & PCA Part B5):

• Discharge over a tundish with a visible air gap (minimum twice the discharge pipe diameter, usually 25–50 mm).
• Tundish must connect to a trapped waste gully or floor waste (graded pipe, no trap in the discharge line itself).

Trap tundish vs in-wall tundish — which to use?

Trap tundish (with built-in trap/seal):

• Preferred for hot water systems.
• Reece example: Allproof 50mm Hot Water Cylinder Tundish (code 1309033, $20.25 ex GST) — cheap, compliant, and has the trap seal.

In-wall tundish (concealed, no built-in trap):

• Acceptable only if it discharges to a trapped waste point downstream (e.g., floor waste gully).
• Reece examples: Zurn In Wall Stainless Steel Face with Clear Plastic Window (code 4000430, $76.89) or Allproof Inwall GPO Tundish 50mm (code 1309036, $18.59).

Best practice: Use a trap tundish for hot water system.

Quick Estimating Checklist for RPZD Jobs

• RPZD unit (e.g., Wilkins 009, Watts 009) + annual testing allowance ($200–$500/year)
• Tundish: 1 per RPZD relief port (trap type preferred)
• Discharge pipe: 20mm PVC/poly to trapped waste gully
• Total allowance: $800–$2,000 (materials + install + testing)

At SNZ Plumbing Estimating, we include RPZD and correct tundish setup on every high-hazard job — no contamination risks, no compliance failures.

Have you installed RPZDs lately? Which tundish type do you prefer? Comment below or DM us for a quick check on your next quote!

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