OSD + Rainwater Tanks: How Combined Systems Work in NSW

Two tanks, one hole in the ground?

Most new residential projects in NSW end up needing two water systems: a rainwater tank for reuse (driven by BASIX commitments for toilets, laundry and irrigation) and an on-site detention (OSD) system for stormwater (driven by council DCP requirements). On a tight Sydney lot, finding room for both is painful. The obvious question follows: can one tank do both jobs?

Often, yes — but only if it’s designed properly, and only if your council accepts the arrangement. Here’s how combined detention/retention systems actually work.

Detention vs retention: the critical difference

The two functions sound similar but behave in opposite ways:

• Retention (reuse) storage wants to be full. The whole point of a rainwater tank is to capture water and keep it for use. A full tank is a successful tank.
• Detention storage must be empty. OSD only works if the volume is available when the storm arrives. A detention tank that’s already full provides zero flood protection.

This is why you can’t simply point at your 5,000-litre rainwater tank and tell council it’s your OSD. If it rained last week and nobody flushed many toilets since, that tank is full — and the design storm will pass straight through it as if it weren’t there. Councils know this, which is why a plain reuse tank generally earns no detention credit.

How a combined tank solves the problem

A combined detention/retention tank stacks the two functions vertically in one structure:

The retention zone (bottom)

The lower portion of the tank stores water for reuse. The pump draws from this zone for toilets, laundry and garden taps. It fills from roof runoff and stays as full as demand allows.

The detention zone (top)

Above the reuse storage sits the detention air space. A controlled outlet — typically an orifice — is set at the top of the retention zone. In a storm, once the tank fills past that level, water rises into the detention zone and discharges through the orifice at no more than the permissible site discharge (PSD). After the storm, the detention zone drains back down to the orifice level and is empty again, ready for the next event. The reuse water below the orifice never interferes with the detention function, because the detention volume is the air space above the outlet, guaranteed available regardless of how full the reuse zone is.

Add a trash screen on the outlet, an overflow for events bigger than the design storm, and access for maintenance, and you have one structure doing two jobs.

What councils accept — and what they scrutinise

Acceptance varies across NSW councils, so treat this as a design conversation, not a default. In our experience councils focus on a few things:

• The detention volume must be dedicated. Only the air space above the orifice counts toward the site storage requirement (SSR). Councils will not credit the reuse volume, though some allow a partial credit for retention under their own rules — check the current DCP or OSD handbook.
• The hydraulics must stand alone. PSD and SSR are calculated exactly as for a conventional OSD tank; the reuse zone is simply ignored in the detention calculations.
• Levels and overflow. The detention zone must drain by gravity through the orifice and the overflow must reach the lawful discharge point safely.
• Documentation on title. The combined tank usually still requires a positive covenant, and the works-as-executed survey must verify the orifice level and detention volume — not just the total tank size.

Pros and cons for builders

Why combine

• One excavation, one structure, one set of lids — usually cheaper than two separate tanks.
• Saves precious site area on duplexes, granny flat lots and infill builds.
• Single roof drainage system feeding one tank simplifies the hydraulics.

Watch-outs

• The combined tank is bigger and deeper than either tank alone — check rock, groundwater and services before committing.
• The orifice level is sacred. A plumber relocating the outlet during construction can quietly delete your detention volume — and fail your certification.
• BASIX reuse sizing and council SSR are calculated under different frameworks; the tank must satisfy both independently.

Getting it through council

The combined approach succeeds when it’s engineered and documented like a proper OSD system: clear long-section showing the orifice level, separate volumes labelled, PSD/SSR calculations to the council’s adopted method, and pipework to AS/NZS 3500.3. Where a council’s handbook is silent on combined tanks, an early phone call or pre-lodgement query beats a refusal later.

Contrive Consultants designs council-ready OSD and stormwater systems across Sydney and NSW — including combined detention/retention tanks that satisfy BASIX and council in one structure. See our services, call +61 497 848 111, email info@contriveconsultants.com.au or contact us today.