Grease Traps for Commercial Kitchens – Sizing & Compliance 2025

Grease traps (also called grease interceptors) are mandatory in most commercial kitchens in Australia to prevent fats, oils and grease (FOG) from blocking sewers and causing environmental damage. Incorrect sizing or installation leads to frequent blockages, council fines, Sydney Water penalties and expensive clean-outs.

Here is how sizing actually gets decided in Australia, what the authority will and will not accept, and how to price it so the tender still holds when the approved size comes back.

01When Grease Traps Are Mandatory

A grease arrestor is required wherever fats, oils and grease are generated, and the trade waste authority for the address decides both that and the size:

• Required for any commercial kitchen, restaurant, café, takeaway, food court, hotel kitchen, aged care kitchen or food processing facility where fats/oils/grease are generated.
• Exemptions are rare (e.g., very small café with minimal cooking) — most councils/Sydney Water require them for any business with a grease-producing fixture (deep fryer, grill, wok, etc.).
• NSW/VIC/QLD — Sydney Water, Melbourne Water and Queensland Urban Utilities mandate grease traps for new fit-outs or major alterations.

02Who Actually Sizes the Grease Trap

This is the part that trips up estimators, so it is worth being blunt about it. In Sydney Water’s area the grease arrestor is sized by the water authority through trade waste, not by you and not by a table in AS/NZS 3500.2. The trade waste application is the gate. Until it is approved you do not have a size, and anything you put in a quote before that is an allowance.

Two things follow from that, and both affect the number:

• Sizing is driven by maximum hourly flow, not by a fixture-unit lookup. Sydney Water’s guidance is to size the unit to meet your maximum hourly flow, which depends on what is actually draining to it and how the kitchen runs, not just how many fixtures are on the plan.
• The listed units start at 1 000 L. Sydney Water’s listed pre-treatment directory runs from 1 000 L through 1 500 L and on to the 2 000 to 5 000 L range. There is no 200 L or 400 L option on that list. If you have been allowing a few hundred litres for a small café, that allowance will not survive trade waste.

Outside Sydney Water, the same principle holds with a different authority. Hunter Water, Melbourne’s retailers, Queensland Urban Utilities and regional councils each publish their own trade waste sizing requirements. Take the size from whichever authority will be issuing the trade waste agreement for that address.

What to do at tender stage. Price a realistic minimum against the listed sizes, state the assumption in writing, and flag that the final size is subject to trade waste approval. That single line protects the margin when the approved size comes back larger than the allowance, which it often does.

03Installation & Compliance Requirements

• Location: Outside building, accessible for maintenance (pumping every 1–3 months).
• Vent: Vent pipe to atmosphere (Clause 6.10).
• Sampling point: Required for oil/grease monitoring.
• Pump-out: Contract with licensed waste contractor.
• Labelling: "GREASE TRAP – DO NOT DISCHARGE CHEMICALS" signs.

04Common Mistakes & Rejections

• Undersizing (e.g., 200 L for a 60-unit kitchen) → frequent blockages & fines.
• No maintenance plan — Sydney Water can issue notices.
• Internal installation (not accessible) → rejected.
• No grease trap on Notice of Work → compliance fail.

05Estimating Tips for Sydney Commercial Kitchens

Because the size comes from the authority, price the installation properly and treat the vessel as a named allowance:

• Start at 1 000 L as the smallest listed unit in Sydney Water’s area. A café or small takeaway is an allowance at that size, not a smaller one.
• A restaurant with a fryer, grill or wok station commonly lands well above the minimum. Two to five thousand litres is a realistic band to carry until trade waste confirms.
• The vessel is rarely the biggest line. Excavation, the concrete surround or pit, the sampling point, reinstatement of the slab or pavement, and getting a heavy unit into a tight back-of-house are usually where the cost sits. Price those against the actual site access, not a rate per litre.
• Carry the trade waste application and the drawings that go with it as their own item. It is real time and it is a precondition, not an afterthought.
• Add the pump-out contract to any maintenance quote. Servicing is typically every one to three months depending on load, and the authority can act if it lapses.
• Write the assumption down. "Grease arrestor allowance based on X litres, final size subject to trade waste approval" is the line that converts a variation into a conversation instead of an argument.

At SNZ Plumbing Estimating we price the grease arrestor as an allowance against the authority’s listed sizes, with the excavation, pit and reinstatement itemised, so the tender holds when the approved size lands.

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