AS 3500 Plumbing Code: Complete 2026 Commercial Guide

What You Need to Know

AS/NZS 3500 is the standard series that governs plumbing and drainage design across Australia and New Zealand. For commercial buildings, it covers everything from cold and hot water supply to sanitary drainage, stormwater systems, and heated water services. If your project needs a Construction Certificate or Complying Development Certificate, the hydraulic design must comply with AS 3500.

The standard is split into six parts. Part 3500.0 covers general terminology. Part 3500.1 sets requirements for water services, including pipe sizing, pressure, and backflow prevention. Part 3500.2 covers sanitary plumbing and drainage. Part 3500.3 deals with stormwater drainage. Part 3500.4 addresses heated water services, including Legionella prevention. Part 3500.5 applies specifically to housing installations and is generally not relevant to commercial projects.

For commercial buildings, a hydraulic engineer produces the design documentation. A licensed plumber installs the system. The certifier checks that the design complies with AS 3500 and the NCC 2025 (which references AS 3500 directly). Getting the design wrong means failed inspections, costly rework, and potential health risks from issues such as cross-contamination or Legionella growth.

• Water supply pipes must be sized to deliver minimum flow rates and pressures at all outlets simultaneously. AS 3500.1 requires that the most hydraulically disadvantaged fixture receives a minimum of 150 kPa residual pressure during peak demand. Pipe sizing uses probabilistic demand calculations based on fixture unit loading. (AS/NZS 3500.1, Section 3 and Section 4)
• Backflow prevention devices must match the hazard rating of the connected system. AS 3500.1 classifies hazards as high, medium, or low. High-hazard connections (such as fire services, cooling towers, and chemical dosing) require reduced pressure zone devices (RPZD). Medium-hazard connections require double check valves. Incorrect device selection is the most common compliance failure. (AS/NZS 3500.1, Section 4)
• Sanitary drainage pipes must be sized using fixture unit loading and installed at minimum grades. AS 3500.2 specifies minimum grades of 1 in 60 for 100 mm pipes and 1 in 90 for 150 mm pipes. Every trap must be protected by a vent to prevent seal loss from siphonage or back pressure. (AS/NZS 3500.2, Section 3 and Section 5)
• Stormwater systems must be designed for the appropriate Average Recurrence Interval (ARI). AS 3500.3 requires a minimum 20-year ARI for most commercial buildings, with 100-year ARI for overflow provisions. Roof drainage, gutters, downpipes, and site drainage all have separate sizing methods. (AS/NZS 3500.3, Section 3)
• Hot water storage systems must maintain water at 60 degrees Celsius or above to prevent Legionella growth. AS 3500.4 requires delivery temperatures of no more than 50 degrees Celsius at sanitary fixtures accessible to the public. Tempering valves or thermostatic mixing valves are mandatory at point of use. (AS/NZS 3500.4, Section 2 and Section 3)
• Trade waste discharge requires pre-treatment before connecting to the sewer. Grease traps are mandatory for commercial kitchens. Other trade waste, including cooling tower blowdown, laboratory waste, and dental surgery waste, must comply with the local water authority's trade waste policy. In Sydney, this is Sydney Water's trade waste requirements. (AS/NZS 3500.2, Section 8; Sydney Water Trade Waste Policy)
• NCC 2025 references AS 3500 as the primary standard for plumbing compliance. The Plumbing Code of Australia (PCA), contained within NCC Volume Three, adopts AS 3500 as a Deemed-to-Satisfy solution. State plumbing regulations (in NSW, the Plumbing and Drainage Act 2011) add additional requirements for licensing, inspections, and approvals. (NCC 2025 Volume Three; Plumbing and Drainage Act 2011 NSW)

Water supply sizing is the first step in any commercial hydraulic design. The hydraulic engineer calculates the probable simultaneous demand based on the number and type of fixtures in the building. A 10-storey office building with amenities on every floor has a very different demand profile to a ground-floor restaurant with a commercial kitchen. AS 3500.1 uses fixture unit values and probability factors to determine the peak flow rate, then the pipes are sized to deliver that flow at the required pressure to every outlet.

Pressure is the constraint that catches most projects. The mains water supply in Sydney typically delivers 200 to 350 kPa at the meter, depending on location. For a multi-storey building, static head loss alone (approximately 10 kPa per metre of height) means the upper floors may not receive adequate pressure without a booster pump system. The hydraulic engineer must calculate whether a pump is needed and, if so, specify the pump duty, control arrangement, and storage requirements.

Sanitary drainage design requires careful coordination of pipe grades, vent connections, and stack sizing. In a multi-storey building, the drainage stack must be sized to handle the combined discharge from all floors without exceeding the stack's capacity. Venting prevents trap seal loss, which would allow sewer gases to enter the building. AS 3500.2 allows several venting methods, including individual vents, circuit vents, and stack vents. The choice depends on the building layout, the number of fixtures, and the distance from the stack.

Stormwater drainage is often underestimated. AS 3500.3 requires the system to handle a 20-year ARI storm event without flooding. For a 1,000 square metre commercial roof in Sydney, this means designing for a peak rainfall intensity of approximately 200 mm per hour. The roof drainage, box gutters, downpipes, and underground drainage must all be sized accordingly. Overflow provisions for a 100-year ARI event must also be incorporated. Failure to size stormwater correctly leads to roof ponding, internal flooding, and structural damage.

Hot water systems in commercial buildings require particular attention to Legionella prevention. Legionella bacteria thrive in water between 20 and 45 degrees Celsius. AS 3500.4 mandates storage at 60 degrees Celsius minimum, with distribution temperatures managed by tempering valves to prevent scalding. For large buildings with long pipe runs, recirculating hot water systems are needed to ensure hot water reaches distant fixtures quickly and to prevent dead legs where stagnant water can harbour bacteria.

Trade waste is a compliance area that many developers overlook until late in the project. Any commercial premises that discharges waste other than domestic sewage requires a trade waste agreement with the local water authority. In Sydney, Sydney Water requires a formal application, and the premises must install appropriate pre-treatment. A restaurant needs a grease arrestor sized to the kitchen's maximum discharge. An automotive workshop needs an oil separator. A dental surgery needs an amalgam separator. The hydraulic engineer specifies these devices and their location in the design.

Backflow prevention is non-negotiable. Every connection to the potable water supply that presents a contamination risk must have an appropriate backflow prevention device. The most common error is installing a device that does not match the hazard rating. A cooling tower connection is a high-hazard risk and requires an RPZD, not a double check valve. A garden hose connection is a medium-hazard risk and needs at minimum a dual check valve with atmospheric port. The hydraulic engineer assesses every connection point and specifies the correct device in the design documentation.