Hydraulic vs structural engineer for stormwater — which do I need for my Sydney DA?
The hydraulic / civil engineer designs the water.
For a Sydney DA, the stormwater management plan and on-site detention (OSD) sizing are designed by a hydraulic (civil) engineer. A structural engineer is needed separately to certify any concrete detention tank, basin or retaining wall that holds water. Most projects need both — they cover different things.
The short version: they do different jobs
The confusion is understandable, because the two roles overlap on the same OSD tank. Here is the clean split most Sydney councils expect.
The hydraulic / civil engineer designs the water. They prepare the stormwater management plan (sometimes called a stormwater concept plan or drainage plan) that goes into your DA. This covers roof and surface drainage, pipe sizing, the point of connection to the public system, and — where the council requires it — the on-site detention system that holds back peak flows and releases them slowly. The hydraulic engineer calculates how big the OSD storage and the permissible site discharge need to be. This is hydrology and hydraulics, not steel and concrete.
The structural engineer designs and certifies the box that holds the water. Once the hydraulic engineer says "you need X cubic metres of detention," someone has to make sure the underground concrete tank, the basin walls or the retaining walls actually stand up under full hydrostatic load and don't leak or crack. That is structural engineering, and many councils — Liverpool, Cumberland, Sutherland and others — explicitly ask for a separate
structural engineer's certificate for the OSD tank or basin as a water-retaining structure.
So for a typical Sydney detached dwelling, dual occupancy or small commercial DA with an OSD requirement, you need a hydraulic engineer for the design and a structural engineer for the tank certification. On larger jobs the same firm usually delivers both under one engagement.
What the standards and NSW rules actually say
Stormwater drainage design — AS/NZS 3500.3:2025. This is the plumbing and drainage standard for roof, surface and subsoil stormwater drainage to the point of connection. It governs gutter, downpipe and pipe sizing, and underpins Volume Three of the National Construction Code (NCC). Roof drainage is sized to a 100-year ARI / 1% AEP event, with eaves-gutter overflow provisions tightened in the 2025 amendments.
OSD and catchment hydrology — Australian Rainfall and Runoff (ARR 2019). OSD sizing and flow estimation follow ARR 2019 (published by Geoscience Australia), combined with your specific council's Development Control Plan (DCP) and OSD code. Most Sydney councils set the OSD performance against a defined storm event (commonly the 1-in-100-year / 1% AEP) and a permissible site discharge — these numbers come straight from the local DCP, which is why the design is always council-specific.
Structural certification of the tank — AS 3600, AS/NZS 1170. A concrete OSD tank, basin or retaining wall is certified to
AS 3600 (concrete structures), with loads to the
AS/NZS 1170 series (AS/NZS 1170.0 general principles, 1170.1 permanent/imposed actions, 1170.2 wind), accounting for hydrostatic pressure at full storage, soil and surcharge loads, and buoyancy. Liverpool City Council's OSD standard, for example, requires the structural certificate to reference AS 3600, AS/NZS 1170 and AS 3500. If steel is involved (grates, access covers, structural framing),
AS 4100 (steel structures) applies.
Who is allowed to sign — Design and Building Practitioners Act 2020 (NSW). For Class 2, 3 and 9c "regulated" buildings (apartments and similar), civil, structural and drainage design work must be done by a
registered design practitioner in the relevant class — there are distinct "civil engineering," "structural engineering" and "drainage" practitioner classes, and compliance declarations are lodged on the NSW Planning Portal. Standard houses and most dual occupancies sit outside the DBP scheme, but the same standards still apply and councils still ask for the certificates.
Quick decision guide
- Standard house / dual occ / small commercial DA with OSD: hydraulic engineer for the stormwater plan + OSD sizing; structural engineer to certify the tank/basin/retaining walls.
- No OSD required (small site, council waiver, or direct gravity connection): usually just a hydraulic/civil stormwater plan — no structural tank certificate.
- Above-ground rainwater/detention tank (proprietary product): the supplier's product may be pre-certified; your engineer confirms it suits the site, connections and discharge.
- Class 2 apartment / Class 9c (aged care): registered DBP practitioners required across civil/structural/drainage, with portal compliance declarations.
Always check your council's DCP stormwater section first — it dictates exactly what to submit and which certificates the consent will require.
Frequently Asked Questions
❯ Do I always need an OSD for a Sydney DA?
No. Whether OSD is required depends on your council's DCP, the catchment, and how much extra impervious area (roof, paving) your development creates. Some sites with a direct gravity connection or below a size threshold are exempt. Your hydraulic engineer checks the DCP and confirms.
❯ Can one engineer do both the hydraulic design and the structural certificate?
Often, yes. A multidisciplinary firm with both civil and structural capability can issue the stormwater management plan and the structural tank certification under one engagement, which keeps coordination tight and turnaround faster. Metric does both in-house.
❯ Is a "hydraulic engineer" the same as a plumber?
No. A licensed plumber installs to AS/NZS 3500. A hydraulic/civil engineer does the design and calculations — catchment hydrology, OSD sizing to ARR 2019 and the DCP, and the stormwater management plan your DA needs.
❯ What does the structural engineer's certificate actually cover?
The structural stability and water-tightness of the OSD tank, basin or retaining walls under full hydrostatic load — designed and certified to AS 3600 and AS/NZS 1170, so the structure won't crack, leak or fail when the detention storage is full.
❯ Does the DBP Act apply to my project?
Only if it's a "regulated building" — generally Class 2, 3 or 9c (apartments, some residential and aged-care buildings). Standalone houses and most dual occupancies are outside the scheme. We'll tell you which applies before you commit.
Reviewed by Youssef Emad, MIEAust 5372671, Registered Professional Engineer (NSW) PRE0002581, RPEQ 37639, Metric Engineering. NSW-based structural and civil engineering — design and certification.