Installing an OSD tank is not a decision you make on the fly. If you are building a new home, undertaking a renovation, or developing a commercial property, understanding how a stormwater detention system works before construction begins can help you avoid costly delays, unexpected expenses, and compliance issues later on.
As Australia’s urban areas continue to grow, more hard surfaces are increasing the volume of stormwater runoff entering local drainage networks. As a result, managing stormwater has become an important requirement for councils across the country.
This blog covers the key factors to consider before installing an on-site detention tank, helping you plan your project with greater confidence and avoid mistakes.
This is the first question to answer, and it depends on where your property is located and what kind of development you are undertaking.
Many councils across New South Wales, Victoria, and Queensland now require OSD tanks as a condition of development approval, particularly in areas with known flood risk or where existing drainage infrastructure is already under pressure.
If you are building in a new subdivision, there is a very good chance an OSD tank will be required. The same applies to renovations or extensions that increase the impervious surface area of your property, such as additional roofing, paving, or driveway space.
The best place to start is by contacting your local council or speaking with a hydraulic engineer who can assess your site and advise whether an OSD system is needed. They can also help you understand any specific design requirements that apply to your development.
Do not assume your property is exempt without checking first.
When you purchase a property, you also take on the responsibilities that come with managing stormwater on that site.
As a property owner in Australia, your responsibilities include:
It is also important to understand where your responsibilities begin and end. While you are responsible for stormwater management within your property, councils and utility providers are responsible for public drainage infrastructure.
If stormwater runoff from your property causes damage to neighbouring land or places additional strain on the local drainage network, you can be held liable for the consequences.
An OSD tank helps reduce this risk by controlling how quickly stormwater leaves your property, ensuring runoff remains within the limits set by your local council.
Getting the size right is critical. An undersized tank will fail to meet council requirements and could cause drainage problems. Meanwhile, an oversized tank wastes money and excavation effort.
Tank sizing is determined by what engineers call the Site Storage Requirement (SSR), which is calculated based on
The PSD is the maximum flow rate at which water can leave your property and enter the council’s drainage network.
This is not a calculation that can be estimated or guessed. Determining the correct detention volume and discharge rate requires detailed hydraulic analysis based on your site’s characteristics and your council’s requirements.
To ensure the design complies with local regulations, you will need a qualified hydraulic or civil engineer to prepare a hydraulic design report.
Once you know how much storage you need, the next consideration is what type of on-site detention tank suits your site.
Underground OSD systems are the most common solution for residential and commercial properties. They are installed below ground to temporarily store stormwater and release it at a controlled rate, helping to meet council drainage requirements while preserving usable space on the surface.
Above-ground tanks are accepted by some councils. They can be an effective detention solution when site conditions allow, but many councils still prefer below-ground systems to ensure most of the property’s runoff is captured and managed effectively.
The conditions of your site directly affect what type of OSD system can be installed. Key site factors to assess include:
The type of soil on your property can affect how an OSD system performs and how it should be installed. Sandy soils allow water to drain more quickly, while clay soils hold water for longer and can make excavation more challenging. A geotechnical assessment can help determine the soil conditions on your site.
The slope of your land plays an important role in how stormwater flows through the system. On some sites, stormwater can drain naturally to the council connection point, while others require a pump.
Before any excavation begins, it is important to identify the location of underground services such as water, sewer, gas, and electricity. Knowing where these services are located helps avoid damage, delays, and additional costs during construction.
Before finalising a design, it is important to confirm that there is enough space on the site to accommodate the system.
Every council in Australia has its own specific requirements for stormwater detention systems. This is not a one-size-fits-all situation.
When researching your obligations, check your local council’s Development Control Plan (DCP) for the specific chapter covering stormwater and OSD.
Some councils in New South Wales require property owners to enter into a formal service agreement with Sydney Water if their OSD system discharges directly into a Sydney Water-owned drainage asset. Under these agreements, you commit to a regular maintenance schedule and annual reporting.
Getting your development application approved the first time starts with understanding your council’s requirements before construction begins.
Installing an OSD tank is not a set-and-forget arrangement. The system requires regular maintenance to keep it functioning correctly and to meet your obligations as a property owner.
Maintenance for a stormwater detention system includes:
Scheduling a professional inspection at least once a year is the standard recommendation for most residential installations.
Installing an on-site detention tank is a commitment, but it is also one that protects your property, your neighbours, and the broader community from the impacts of uncontrolled stormwater. Taking the time to get the details right before you start will pay off at every stage of the process.
If you need expert advice on planning and installing an on-site detention system, contact Buildaroo today on 0480 838 057 or complete our online form to discuss your project.
