OSD Certification in NSW: WAE Surveys & 88B Instruments

The paperwork between a finished tank and a finished job

You’ve built the OSD tank to the approved drawings, the orifice plate is in, and the site drains beautifully. You’re not done. In NSW, an on-site detention system isn’t complete until it’s certified, surveyed and protected on title — and these conditions of consent regularly hold up occupation certificates because nobody planned for them. Here’s what each piece of the certification puzzle is, and how to get through it without delay.

Works-as-executed (WAE) survey

A works-as-executed survey — often called a WAE or works-as-built survey — is a survey of the OSD system as it was actually constructed, prepared by a registered surveyor and typically overlaid on the approved design drawings. It records the things that determine whether the system actually performs:

• Tank or basin dimensions and the achieved storage volume versus the required site storage requirement (SSR);
• Invert and surface levels — tank floor, orifice level, overflow weir, pit levels and the discharge point;
• Location and size of pipes, pits and the discharge control structure;
• Orifice diameter as installed.

Council (or the certifier) compares the WAE against the approved design. Small construction tolerances are normal; a tank 15% short on volume or an orifice at the wrong level is not. The brutal economics: a WAE survey discovering a problem after the driveway is poured is the most expensive possible time to find it. If anything changed during construction — and on real sites, something usually does — get the engineer to check the impact before you bury the evidence.

Engineer’s certification

Alongside the survey, consents typically require certification from a suitably qualified civil/hydraulic engineer that the OSD system has been constructed in accordance with the approved design and will function as intended — covering the storage volume, the discharge control (orifice and any high early discharge arrangement), the trash screen, overflow provisions and the drainage connections, with pipework compliant with AS/NZS 3500.3. The engineer relies on the WAE survey plus inspection, which is why it pays to use the same engineer who designed the system and to involve them before critical elements are covered up.

Positive covenants and 88B instruments: protecting the system forever

Here’s the part that surprises many builders: the OSD system doesn’t just need to be built correctly — it needs to be legally protected so future owners can’t remove or neglect it. A detention tank only protects the catchment if it still works in thirty years, under an owner who’s never heard of OSD.

What a positive covenant is

A positive covenant is a legal obligation registered on the title of the land under the Conveyancing Act 1919, requiring the owner (and every future owner) to maintain the OSD system in working order. Unlike a restriction, which stops an owner doing something, a positive covenant requires them to do something — keep the tank clear, the screen clean and the system functioning. Council is typically the authority benefited, with power to enforce.

Where the 88B instrument fits

When land is being subdivided, easements, restrictions and positive covenants are created through a Section 88B instrument — the legal document accompanying the deposited plan that sets out these interests. On subdivisions (duplex torrens subdivisions, multi-lot estates), the OSD positive covenant and any drainage easements are usually created via the 88B at plan registration. On developments without subdivision, the covenant is created by separate dealing registered on the existing title. Either way, councils generally have standard terms they require word-for-word — using council’s template wording avoids a rejection cycle at the land registry.

Sequencing: how this fits around your occupation certificate

A typical consent sequences it like this:

• During construction — hold points or inspections on the tank, pipework and discharge control pit (check your consent; some councils want to see the orifice before backfill).
• At completion — WAE survey prepared; engineer inspects and certifies; defects rectified.
• Before the occupation certificate (or subdivision certificate) — WAE and certification submitted and accepted; positive covenant registered (via 88B on subdivisions); evidence of registration provided to the certifier.

The choke point is almost always the covenant: title dealings involve solicitors or conveyancers, council execution and land registry processing, none of which happen in a day. Start the covenant paperwork when the tank is built, not when the OC application is lodged.

Builder’s checklist

• Read the OSD conditions of consent at project start — note every certification deliverable.
• Photograph and level-check the orifice and pit before backfill.
• Book the WAE surveyor and certifying engineer before practical completion.
• Brief the client’s conveyancer on the positive covenant early.

Contrive Consultants designs council-ready OSD and stormwater systems across Sydney and NSW, and supports WAE review and certification through to occupation certificate. See our services, call +61 497 848 111, email info@contriveconsultants.com.au or contact us today.