Most people never think about stormwater systems until they need one. If your property requires an On-Site Detention (OSD) tank, you might be wondering what it actually looks like and where it is installed. In this blog, we’ll explain the two types of OSD tanks, what they look like, and how they help manage stormwater on your property.
There are two main types of OSD tanks used for residential and commercial properties in Australia: underground and above-ground OSD tanks. Each has its own benefits depending on your site layout and available space.
An underground OSD tank is installed below the ground, beneath a driveway, garage slab, or garden area.
From street level, you would hardly know an underground OSD tank is there. The only visible parts are the access lids, which sit flush with the ground. These openings are generally 900 mm x 900 mm and are spaced about every six metres along the tank to provide ventilation, inspection access, and safe entry.
In driveways or other trafficable areas, cast iron lids are used. In garden or pedestrian areas, poly lids are more common. When installed under a driveway, the finished surface looks just like a normal driveway, with only the lid surrounds showing what is underneath.
An underground OSD tank is a large rectangular or square reinforced concrete box buried below the ground. Inside, it is hollow so it can temporarily store stormwater during heavy rain before releasing it slowly afterwards.
The walls are made from high-strength concrete, commonly 50 MPa, reinforced with steel bars and mesh to withstand surrounding soil pressure and any vehicle loads above. The base is a structural concrete slab, while the roof is a flat concrete panel built to carry vehicle loads where needed.
Inside the tank, you will find the inlet pipe where stormwater enters, the orifice outlet, which is a small engineered opening fitted with a mesh trash screen to prevent debris from blocking it, and an emergency overflow connection that operates when rainfall exceeds the tank’s capacity.
Concrete is the most common material used for underground OSD tanks, and councils specifically require it for below-ground installations. Many projects use precast concrete tanks, which are built off-site and lifted into an excavated pit by crane, making installation much quicker than constructing a tank on-site.
Common residential sizes range from 6,700 litres to 22,500 litres in a single precast unit, with modular systems available for larger capacities. Some councils and sites also allow high-density polyethylene (HDPE) or corrugated steel tanks with a polyethylene liner, depending on the local Development Control Plan.
The main maintenance tasks are keeping the orifice outlet free of debris and making sure the trash screen does not become blocked. Access should only be carried out by a licensed professional because entering a below-ground tank involves significant safety risks. Routine inspections every six to twelve months are common industry practice.
An above-ground OSD tank is installed on the surface. It is a practical and cost-effective option for many residential properties and, when positioned carefully, can blend into the landscape without standing out.
Above-ground OSD tanks are visible, and their appearance depends on the material used. Polyethylene (poly) tanks are the most common choice for homes. They look similar to standard rainwater tanks, with a rounded or cylindrical shape.
They sit upright on a flat base and include a screened inlet at the top, an outlet pipe near the bottom fitted with the engineered orifice plate, and an overflow connection near the upper section of the tank.
Steel tanks have a corrugated cylindrical design and a more industrial appearance, making them better suited to commercial or higher-capacity applications. Concrete above-ground OSD tanks are square or rectangular and can be rendered, painted, tiled, or clad with timber or stone to match the surrounding landscape.
Most above-ground OSD systems also include a small control pit or outlet structure nearby. This is where the orifice plate is located and where the discharge pipe connects to the public stormwater system. The orifice plate is a small circular or rectangular opening covered with a fine mesh screen to stop debris from blocking the outlet.
A well-designed installation places an above-ground OSD tank in a discreet location, such as along a fence line, under a deck, beside a retaining wall, or in a corner of the yard where it blends into the landscape.
Poly tanks can be screened with trellis panels, garden beds, or timber fencing to reduce their visual impact. Concrete tanks can be finished to match the exterior of the home.
Above-ground OSD tanks are commonly made from polyethylene, steel, or concrete. Poly tanks are lightweight, resistant to corrosion, and easy to install. Steel is well suited to commercial or high-use environments. Concrete tanks offer the longest service life and the greatest flexibility for blending into landscaping, although they require crane installation.
One of the biggest advantages of above-ground systems is easy access for maintenance. Because the tank is at ground level, inspections, debris removal, and outlet checks can be completed without excavation.
OSD tanks play an important role in managing stormwater and meeting local council requirements. Whether your property needs an underground or above-ground system, understanding what they look like can help you make better decisions.
If you need help designing or installing an OSD tank, Buildaroo can provide advice and reliable construction services. Contact our team on 0480 838 057 or use our online form to discuss your project.
