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Guide · Rainwater

Underground rainwater tanks in Australia

Underground rainwater tanks store harvested roof water below ground so you keep capacity without a bulky aboveground vessel. In Australia, GRAF Carat and Platin tanks are the primary modular options for homes and commercial sites. Size to catchment and demand, then buy and quote through GRAF Australia (grafaustralia.com.au, 1300 131 971) for local supply, Reece/trade channels and install guidance. graf.info is for global engineering reference, not AU project quotes.

GRAF underground rainwater tank installation on an Australian project

When underground tanks make sense

Choose underground storage when space is limited, aesthetics matter, or you need large volumes that would dominate a yard or car park. Burial protects water from UV and keeps the footprint clear for landscaping or pavement.

Typical Australian uses include irrigation, toilet flushing, laundry and mixed non-potable reuse where plumbing and council rules allow. Confirm potable vs non-potable requirements with your plumber and authority.

GRAF Carat and Platin vs retail DIY tanks

Retail underground listings often target DIY and small domestic installs. GRAF Carat and Platin are engineered modular platforms specified on residential and commercial projects, with AU documentation, load guidance and state sales support.

Carat is the core modular family. Platin suits flatter profiles or shallow excavation constraints. Pair tanks with filters and pumps so the system delivers reliable water quality and pressure.

Sydney, Melbourne, Brisbane, Perth and Adelaide

Demand is strong in major cities, but soil, groundwater and council reuse rules differ. Western Australian sandy sites, Melbourne clay and coastal covers all affect bedding and load design.

GRAF Australia manufactures rotomoulded products in Western Australia and supplies nationally. Send postcode, roof area and reuse intent with your quote so the right state team can respond.

Sizing and next steps

Start with roof area, rainfall and intended reuse. Design tools on this site give a first pass. Then request a quote for fittings, filters, pumps and lead times.

If your brief is stormwater OSD or detention rather than reuse, use the OSD and stormwater detention guide instead.

Related products

Frequently asked questions

What is the best underground rainwater tank in Australia?+
The right tank depends on capacity, excavation depth, traffic load and reuse demand. GRAF Carat and Platin underground tanks are widely specified for Australian homes and commercial sites because they are modular, engineered for burial, and supported locally by GRAF Australia for sizing, supply and documentation.
How big should a rainwater tank be for a house in Australia?+
Sizing depends on roof catchment, local rainfall and how you will use the water. Many Australian homes start in the multi-thousand litre range; commercial sites go higher. Use GRAF Australia design tools or request a quote with roof area and postcode.
Carat vs Platin underground tanks: what is the difference?+
Both are GRAF underground platforms sold in Australia. Carat is the core modular family for many rainwater and related roles. Platin is often preferred where a lower profile or particular layout suits the site. GRAF Australia recommends based on depth, footprint and application.
Are underground plastic tanks better than concrete?+
Polyethylene modular tanks are lighter, often faster to install and easier to stage on constrained sites. Concrete can suit some civil specs. Compare load class, cover depth, access and programme with GRAF Australia for your project.
Can I buy GRAF underground tanks through Reece?+
Many Reece branches stock or can order GRAF products. For project quotes, custom configurations and engineering support, contact GRAF Australia at grafaustralia.com.au or 1300 131 971.
Do underground rainwater tanks need pumps and filters?+
Most reuse systems need filtration plus a pump or pressure system to deliver water to fixtures. GRAF supplies matching filters and accessories. Specify them with the tank as a package.
Underground rainwater tank in Sydney, Melbourne, Brisbane or Perth?+
GRAF Australia supplies nationally with state sales support. Soil, cover depth and council reuse rules vary by city. Share your postcode and plans on the quote form and we will confirm the right Carat or Platin configuration.
Is an underground rainwater tank the same as an OSD tank?+
No. Rainwater tanks store water for reuse. OSD (on-site detention) tanks control stormwater discharge rates. See our OSD and stormwater detention guide if your consent condition is detention, not harvesting.

Need a quote for your Australian site?

Call 1300 131 971 or send project details — GRAF Australia will size and price the right system.

Request a quote