OSD tanks in Geelong: rules, sizing and structural design

Stormwater design

Updated 30 October 2026 · 7 min read

In short: In Geelong, you typically need an OSD tank (on-site stormwater detention tank) when a multi-unit or other development will send more runoff to an existing drain than it was designed for. The tank holds runoff from roofs and paving and releases it slowly through an orifice, at no more than council's permissible site discharge. The City applies the Infrastructure Design Manual (IDM) and its own Design Note 2.

Key takeaways

An OSD tank stores runoff and releases it through an orifice limited to the permissible site discharge (PSD), with an overflow for big storms.
Geelong reads its Design Note 2 with the IDM; where the two conflict, the Design Note takes precedence.
For sites up to about 1 ha with no council-set PSD, IDM Table 13 gives the discharge rate and the storage per square metre.
Detention releases stormwater slowly through an orifice; retention keeps rainwater for toilets, washing machines and gardens. One tank can be set up for both.
A buried tank is checked for soil, any traffic and groundwater uplift; when specific calculations are submitted, the IDM asks for its structural computations.

A dark grey rectangular underground stormwater tank with two black access risers, sitting on a bed of crushed grey rock in a timber-edged excavation of red-brown soil, beside a concrete path and a red brick house

Building units or townhouses in Geelong with a stormwater detention requirement? Get a quote or call 03 4250 8105, and send your site plan and planning permit.

What is an OSD tank and how does it work?

An OSD tank is on-site stormwater detention: a tank, pipe or pit system that holds runoff from a site's roofs and paving during a storm and releases it slowly to the council drain. The Infrastructure Design Manual (IDM), a joint initiative of Victorian rural and regional councils, says on-site detention aims to stop developments that add runoff from exceeding the capacity of existing drains.

An OSD tank has three working parts:

Storage. The tank holds runoff that arrives faster than the outlet can release it, then empties slowly, as Graf's technical guide for its Platin tank (sold in Australia through Reece) describes.
Outflow control. The IDM lists a pipe, an orifice plate or a proprietary multi-cell unit as typical outflow controls. An orifice plate is a plate with a small hole sized for the permissible site discharge (PSD), the outflow rate council will accept. Graf's orifice table shows the flow through an orifice rising as the water above it gets deeper.
Overflow. The IDM says a detention system should have an overflow for storms rarer than its design storm, up to and including the 1% Annual Exceedance Probability (AEP) event, and for blockages. A 1% AEP storm has a 1% chance of being exceeded in any year. Overflows should be directed away from buildings and neighbouring properties, and council will expect the designer to show the maximum water level stays at least 300 mm below the lowest floor of any home in the overflow zone.

Cross-section of an underground OSD tank: inlet from roofs and paving, storage, the depth of water above the orifice, an orifice plate limiting outflow to the permissible site discharge, the outlet to the legal point of discharge, an overflow directed away from buildings, and an access cover rated for the loads above — How an OSD tank works: storage fills during a storm, the orifice plate limits the outflow to the permissible site discharge (PSD), and the overflow handles bigger storms or a blocked orifice. Indicative only; sizes and levels come from the engineer's design.

When does the City of Greater Geelong require an OSD tank?

The City of Greater Geelong may require an OSD tank when a development will send more stormwater to an existing drain than that drain was designed to carry. Greater Geelong is one of the IDM's participating councils, and IDM Clause 19.2.1 lists the developments that typically need on-site detention:

multi-unit development in newer residential areas where the drainage design made no specific provision for it;
multi-unit development in older residential areas where the drains were designed for a peak flow significantly lower than the IDM's runoff coefficients give for a 20% AEP storm;
industrial and major commercial development where the drains were designed for significantly lower peak flows;
low-density residential development within or next to urban or rural township areas.

The IDM says on-site detention will not usually be required on rural lots over 2 ha, unless measures are needed to protect streams or constructed waterways from erosion.

The IDM raises on-site detention at the planning permit stage. Under Clause 3.3.4.3, even when council does not ask for a Stormwater Management Strategy, the developer should show that stormwater can reach the existing drains without harming how they operate, and this may require significant on-site detention.

The legal point of discharge (LPOD) is the discharge point council nominates for a site's stormwater. Under IDM Clause 21, council identifies legal points of discharge for all developments and may first assess the impact on existing drains; where a development is likely to affect them adversely, the works needed may include on-site detention.

How is an OSD tank sized in Geelong?

An OSD tank in Geelong is sized so the site's outflow at the LPOD stays within the PSD. Geelong's Design Note 2, revised in January 2023, says private lot-scale detention uses IDM Clause 19, Table 13 to determine the PSD at the LPOD. The note is read with the IDM, and where the two conflict, the note takes precedence.

Table 13's simplified method applies to sites of up to about 1 ha where council has not specified the PSD. Its multi-unit rows are:

Multi-unit development (IDM Table 13)	Allowable discharge rate	Storage in a vertical tank or basin	Storage in horizontal pipes
Newer residential areas	65 litres per second per hectare	8 litres per m²	10 litres per m²
Older residential areas	37 litres per second per hectare	12 litres per m²	14 litres per m²

In the IDM's worked example, a multi-unit development on a 750 m² site in an older area sends 60% of its runoff to rainwater tanks and 40% to underground pipes. Table 13 then asks for 0.6 × 750 × 12 = 5,400 litres in the tanks and 0.4 × 750 × 14 = 4,200 litres in the pipes.

Specific calculations by a qualified civil engineer are required when council's nominated peak discharge rates differ significantly from those the table assumes. For private detention on sites of up to 10 ha, Design Note 2 accepts the Modified Rational Method or hydrological routing. The IDM's council checklist then asks whether the orifice size has been calculated and whether storage is spread between pipes, pits and tanks in proportion to where the flow comes from.

Which OSD tank type suits the site?

The IDM's most commonly acceptable on-site detention systems include rainwater tanks, underground pipes and tanks, kerbed driveways and pumped systems, and the choice depends partly on site levels and the loads above.

OSD option	What the sources say	Points to check
Above-ground rainwater tank	The IDM lists conventional rainwater storage tanks as an acceptable system	The IDM checklist asks whether the outlet could be made larger by making the tank lower and wider
Below-ground tank or pipes under a garden	The IDM lists underground pipes and tanks of various configurations	Graf's deepest installation is for green areas only, unless an engineered suspended slab is installed
Tank under a driveway	Graf's car-rated set-up uses a cast cover, with the riser collar supported on concrete; the IDM asks for pit covers rated for the potential traffic loads	Graf's car-rated set-up is for passenger cars up to 3.5 tonnes; surfaces used by heavier vehicles stay at least the trench depth away
Kerbed driveway storage	The IDM accepts driveways bounded by kerbs at least 100 mm wide	The IDM asks for evidence that habitable floors are at least 300 mm above the peak water level

What loads does a buried OSD tank have to carry?

A buried OSD tank has to carry the soil around and above it, any vehicles over or beside it, and the upward push of groundwater when it is empty. When specific calculations are submitted, the IDM asks for structural computations for any underground storage tank and cross-sections of each storage device.

Soil and slopes. Before installation, Graf's guide asks for checks of the ground's structural suitability (it recommends a geotechnical report), the maximum groundwater level and the expected loads, such as traffic. A tank within 5 m of a slope or mound needs a statically calculated supporting wall to take the soil pressure.
Groundwater uplift. Contech, a US manufacturer, explains that an empty tank in saturated ground is pushed up by the weight of the water it displaces. Soil cover, the tank's own weight, a reinforced concrete pad above it or concrete anchors can hold it down. Graf sets a maximum groundwater immersion depth for each tank size and recommends draining water away from tanks in clay or other water-tight soils.

What is the difference between detention and retention?

Detention holds stormwater during a storm and releases it slowly at a controlled rate; retention keeps rainwater on site for use.

	Detention (OSD)	Retention (rainwater reuse)
Purpose	Limit the site's peak outflow to the PSD (IDM Clause 19)	Reduce the runoff volume and pollutant loads leaving the site (IDM Clause 20.3.10)
After the storm	Drains slowly through the outflow control	Stays in the tank for flushing toilets, filling washing machines and watering gardens (Graf)

One tank can do both. Graf's guide says a correctly sized and set-up tank can detain the volume council requires and also retain rainwater for use, and it ties the detention volume to the orifice height in the tank.

When to call a structural engineer

Call an engineer as soon as council or a planning permit asks for on-site detention, because the IDM expects the designs and calculations to come from a qualified civil engineer and to be approved by council's engineering department. Call one early when:

the tank will sit under a driveway or car park, or near heavy vehicles;
the site has high groundwater, clay soils, or a slope within 5 m of the tank;
one tank is meant to handle both detention and rainwater reuse.

Our civil engineering page covers design and documentation for on-site detention (OSD) and on-site retention (OSR) systems. Our residential stormwater page covers drainage designs for single dwellings and multi-unit developments, which may include detention systems, rainwater storage and pumps. Our page on stormwater and drainage issues causing structural damage explains how water collecting around footings can soften the soil and lead to settlement and cracking.

Frequently asked questions

Who maintains an OSD tank after the units are sold?

The landowners do. The IDM says council and the developer may enter a Section 173 Agreement that binds future landowners to keep the system working, give council officers reasonable access to inspect it, and possibly pay an annual inspection fee. Its standard agreement also says the owner must not modify the system without council's written agreement.

Can council waive the need for on-site detention?

In some cases, yes. IDM Clause 19.3.4 says council's engineering department may waive on-site detention where it can be shown that the extra stormwater from the development causes no adverse impacts and council's level of service will not be compromised.

What if the site sits lower than the street?

A pump may be needed. The IDM says pumping may become the only viable option when a site is below street level and no drainage network runs along the rear boundary. Small pumped systems serving catchments of up to 2,000 m² where stormwater can't reach the legal point of discharge by gravity may be designed to Section 9 of AS/NZS 3500.3; council expects other pumped systems to be designed by a qualified engineer.

What happens if the orifice blocks?

Where the orifice or tube sits on the downstream face of a pit, the IDM expects the designer to deal with a blockage in a design storm. Its options include a safe overland route for the surplus flow, enough storage to keep the flow on site, or a dual-chamber pit with the controlled flow passing an internal weir wall.

What does council need to nominate the point of discharge for a dwelling?

The IDM lists a plan of the proposed development showing its pervious and impervious areas, existing and proposed surface levels at intervals of no more than 10 m, and the location and size of existing drains and easements in and next to the site.

When do you need an OSD tank in Geelong?