Detention/Retention Water Tanks: What You Need to Know
If you’re building or renovating, your council or engineer may specify a detention/retention tank as part of the stormwater management plan. These tanks are designed to reduce pressure on local drainage systems and, in many cases, also provide water for household use.
You’ll often hear the terms detention tank and retention tank used interchangeably — even councils and builders mix them together. What really matters is how the tank is set up and what’s shown on your building plans.
What’s the Difference?
The difference comes down to how the outlets are positioned and what the tank does with water:
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Detention → Holds stormwater temporarily and then slowly releases it into the stormwater system. The tank drains after rain and doesn’t store water for re-use.
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Retention → Stores a permanent supply of water for re-use (garden, toilets, laundry), while also having a detention outlet to release excess water slowly.
Easy way to remember:
Detention = delay and release. Retention = store and re-use.
Outlet Position: The Key Detail
One of the clearest differences between detention and retention setups is the height of the slow-release outlet:
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Detention outlet – fitted at the lowest point so the tank drains almost completely.
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Retention outlet – fitted higher up the wall, so the bottom of the tank always keeps a usable water volume while excess drains away slowly.
In both cases, the slow-release outlet connects into the overflow pipework to release water at a controlled rate into the stormwater system.
Who Decides What You Need?
The size and setup of your detention/retention tank is not a guess — it’s specified by your council, building surveyor, or engineer as part of your building approval. They’ll outline:
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The required tank capacity
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How much water must be retained for use
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The size and position of detention outlets
Installation Requirements
Because these tanks form part of the stormwater system, installation usually needs to be carried out by a licensed plumber. In most cases, the builder or plumber handling your project will arrange both the tank and its installation, ensuring compliance and final sign-off.
Can Any Tank Be Used?
Yes. With the right fittings, any water tank can be converted into a detention/retention system:
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Slow-release outlet with flow restrictor
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Retention outlet and pump for re-use
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Overflow system connection
At Tankworld Tasmania, we can supply tanks and fittings to meet council requirements and work directly with your builder or plumber.
Best Options for Tight Spaces
Detention/retention tanks are most common on urban blocks with limited space. That’s where our Slimline Tanks are ideal — sitting neatly against fences and walls without taking up valuable yard area.
Where space allows, small round tanks are also an effective, budget-friendly option.
Key Takeaways
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Councils and builders often call them detention/retention tanks — the name matters less than the setup.
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Detention = slow release. Retention = re-use + slow release.
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Outlet height is the difference — low for detention, higher for retention.
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The required size and setup is specified on your building plans.
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Installation must be by a licensed plumber to meet compliance.
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Slimline or small round tanks are great solutions for urban sites.
Need help with a detention/retention tank for your project? Talk to our team — we’ll make sure you get the right tank, fittings, and advice to satisfy council requirements.