OSD (on-site detention)
OSD is temporary stormwater storage required by many NSW councils for new dwellings. A hydraulic engineer sizes the tank; council DCP sets the PSD and SSR values.
Ask Chalkline about this →OSD (on-site detention) is a temporary stormwater storage system installed on a property to slow the peak discharge of rainwater runoff into the public drainage network. Instead of letting all roof and hard-surface water rush straight into the street during a storm, an OSD system holds it on site and releases it at a controlled rate.
Many NSW councils require OSD for new dwellings, knockdown-rebuilds, dual occupancies, and extensions that increase impervious area beyond a council-set threshold. For example, Wollongong City Council requires OSD for single dwellings, additions, and dual occupancies, with an exemption only where the increase in impervious area is less than 100 m² (one exemption per site) (verified 2026-05-09). Requirements vary between councils; always check the relevant Development Control Plan (DCP) before designing a stormwater layout.
Key terms:
- PSD (Permissible Site Discharge): the maximum rate at which a site may discharge stormwater to the public system, expressed in litres per second (L/s). Set by the council DCP; the OSD outlet is designed to never exceed this rate.
- SSR (Site Storage Requirement): the minimum detention volume, in cubic metres (m³), that the OSD system must hold. Calculated from local rainfall intensity data and the PSD.
- Discharge control pit (DCP): a pit with a restricted orifice plate that meters the outflow from the OSD tank to the kerb or street drain at or below the PSD.
OSD calculations must be prepared by a qualified civil or hydraulic engineer using Australian Rainfall and Runoff (ARR) methods (verified 2026-05-09). A Certificate of Hydraulic Compliance, certifying the system was built to the approved design, is typically required before an occupation certificate is issued.
System types: common solutions include below-ground concrete or polyethylene tanks, geocellular modular cells under driveways or paving, above-ground basins, and (where soil conditions allow and the council accepts them) infiltration or absorption systems such as soakage trenches. Some councils permit partial OSD offsets using rainwater tanks; the credit percentage and eligibility rules vary by council.
Stormwater plumbing to the OSD system must comply with AS/NZS 3500.3:2025, Plumbing and drainage, Part 3: Stormwater drainage (verified 2026-05-09).
Also known as: on-site stormwater detention, detention tank, OSD tank
Category: Site drainage / stormwater management
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Last updated: 2026-05-09. Verified: 2026-05-09. Quarterly review for currency.