Underground Water Tanks
When space and accessibility are an issue, an underground water tank is the answer, especially in an urban environment.
An increasingly popular option for architects and builders is an underground water tank. This allows homeowners to maximise their space, giving them more green space/or parking.
POLYMASTER UNDERGROUND WATER TANK SOLUTIONS
Browse Our Range of Underground Water Tanks
When placed under paved traffic areas, driveways, garden beds, or lawns, this solution is not only aesthetically pleasing but also an eco-friendly way to capture and harvest rainwater.
Polymaster’s range is available in 3,300L and 5,000L capacities, with a choice of tank-only, vortex filter, or vortex filter and pump configurations. Tanks can be interconnected to increase your holding capacity. Available through 1600+ agricultural and plumbing stores.
| Code | Product | Capacity | Length (mm) | Width (mm) | Height (mm) | ||
|---|---|---|---|---|---|---|---|
 | URT3000VF/PC | 3,300L with Vortex Filter & Household Pump | 3,300L | 2470 | 1640 | 1665 | |
| URT3000VF/GP | 3,300L with Vortex Filter & Garden Pump | 3,300L | 2470 | 1640 | 1665 | |
| URT3000VF | 3,300L with Vortex Filter | 3,300L | 2470 | 1640 | 1665 | |
 | URT3000B | 3,300L Tank Only | 3,300L | 2470 | 1640 | 1665 | Download Drawing |
 | URT5000VF/PC | 5,000L with Vortex Filter & Household Pump | 5,000L | 3520 | 1640 | 1665 | |
| URT5000VF/GP | 5,000L with Vortex Filter & Garden Pump | 5,000L | 3520 | 1640 | 1665 | |
| URT5000VF | 5,000L with Vortex Filter | 5,000L | 3520 | 1640 | 1665 | |
 | URT5000B | 5,000L Tank Only | 5,000L | 3520 | 1640 | 1665 | Download Drawing |
 | U-LID | Lid (Underground Tank) | - | - | - | - | |
 | URRM | 500mm Riser | - | - | - | - |
3,300L with Vortex Filter & Household Pump
URT3000VF/PC
3,300L with Vortex Filter & Garden Pump
URT3000VF/GP
3,300L with Vortex Filter
URT3000VF
3,300L Tank Only
URT3000B
5,000L with Vortex Filter & Household Pump
URT5000VF/PC
5,000L with Vortex Filter & Garden Pump
URT5000VF/GP
5,000L with Vortex Filter
URT5000VF
5,000L Tank Only
URT5000B
Lid (Underground Tank)
U-LID
500mm Riser
URRM
Not sure which option is right for your property?
Our team can help you match the right size and configuration to your requirements. Call us on 1800 062 064 or use the instant quote tool above.
UNDERGROUND WATER TANK APPLICATIONS
Where Can You Install an Underground Water Tank?
One of the biggest advantages of going underground with your water storage is the flexibility it gives you with placement. Because the system sits completely below the surface, you are not restricted to a spare corner of the yard. Polymaster’s range is engineered and load-rated for a wide range of installation locations.
Under the Lawn or Garden
The most common choice for residential properties. Buried beneath a lawn or garden bed, the system is completely invisible once installed and landscaping is finished. There is no impact on your usable outdoor space and no visual disruption to the property.
Under a Driveway or Paved Area
Polymaster’s underground systems are designed for installation under driveways and trafficked areas. The vortex filter unit is rated to handle up to 30 tonnes of vehicle weight, making it suitable for driveways and paved areas where above-ground storage simply is not an option.
Under a Deck or Entertaining Area
For properties where every square metre counts, going under a deck or entertaining area is an increasingly popular choice with architects and builders. The system harvests rainwater from your roof while the space above remains fully usable.
Interconnected Systems for Higher Capacity
Polymaster poly tanks feature a tank-to-tank connection point, which means multiple units can be linked together to increase your total holding capacity without requiring additional footprint. This is a popular solution for residential developments, civil projects, and properties with higher water demand. Talk to our team about interconnected configurations.
Highest Quality and Standards
Every Polymaster poly tank is built to last and independently certified, so you can have complete confidence in what you are putting in the ground.
Food Grade Polyethylene
Certified to AS/NZS 4020, the Australian standard for products in contact with drinking water.
ISO 9001 Quality Management
Polymaster holds ISO 9001 certification, the international standard for quality management systems.
Australian Made
Designed, engineered, and manufactured right here in Australia, built for our climate and our conditions.
Independently Certified
Certified to AS/NZS 4766:2006 for polyethylene storage tanks.
WHY CHOOSE POLYMASTER
Australian-Made Underground Water Tanks Built to Last
Polymaster has been supplying Australian homes, developments, and civil projects with quality liquid storage solutions for over 30 years. Our underground range is not an afterthought – these are purpose-built, geotechnically engineered products designed from the ground up to perform when buried under soil, gravel, and vehicle traffic.
Every unit leaves our facility with independent certification to AS/NZS 4766:2006 and is manufactured from food-grade polyethylene certified to AS/NZS 4020:2018, which means the water you collect is stored safely and is suitable for household use, garden irrigation, and beyond. When you buy from Polymaster, you also receive the Finite Element Analysis calculations used to engineer your system for the required installation depth. No other manufacturer provides this as standard.
FEATURE POINTS
Why Choose Polymaster Underground Water Tanks
There is a lot to consider when choosing a below-ground water storage system. Here is what sets Polymaster apart.
FEA Engineering Calculations Provided
Every unit is engineered by geotechnical experts using Finite Element Analysis. Those calculations come with your purchase – not just a claim, actual documentation.
One-Piece Poly Construction
No joins, no seams, no part lines. Manufactured in a single piece, which eliminates the risk of leaks at connection points and adds long-term structural integrity.
Anti-Flotation Design
Engineering specifications completed to AS1170 address anti-flotation requirements – important for areas with high water tables or where groundwater is present.
Sealed Tamper-Proof Lid
A sealed, locking lid keeps your stored water safe, clean, and secure from the moment the system is commissioned.
SELF-CLEANING VORTEX FILTER
The Self-Cleaning Vortex Filter - Built for Underground Systems
Available as a factory-fitted option across the Polymaster range, the self-cleaning vortex filter is one of the most important components of any underground rainwater harvesting system. It does two things at once – it filters the water coming in from your roof and cleans itself in the process, capturing more than 90 percent of available rainwater while keeping debris and sediment out of your storage.
The unit mounts vertically near your downpipe or beside the tank, suits roof areas up to 200 square metres with 100mm pipe entry, and is available with extension risers for deeper underground setups. For driveways and high-traffic areas, the housing is rated to withstand up to 30 tonnes of vehicle weight, so you never have to worry about what is driving over the top.
INSTALLATION GUIDE
How to Install an Underground Water Tank
Polymaster’s range is designed to make the process as straightforward as possible, with detailed engineering specifications and FEA calculations supplied with every unit. The following overview covers the key stages of a typical below-ground setup. For full specifications, download the Polymaster Underground Water Tank Installation Guide.
Step 1 - Trench Excavation
Excavate to specification with approximately 300mm working clearance on all sides. Ensure safe battering of trench walls, and prepare a firm, level and compacted crushed-rock base for the unit to sit on.
Step 2 - Tank Placement
Lower the unit into position using the lifting lugs. Set it on the prepared base and check it is level in both directions with the feet stable before proceeding.
Step 3 - Pre-Filling
Pre-fill to the specified volume before backfilling begins. This stabilises the unit and counteracts any buoyancy risk if groundwater is present below the midline.
Step 4 - Backfilling
Backfill evenly in layers no greater than 300mm, using 7mm minus or Type B crushed rock. Hand compact each layer carefully without placing stress on the walls.
Step 5 - Plumbing the Feed and Overflow Pipes
Install the inlet and overflow lines with a minimum fall of one per cent. Connect the overflow to the stormwater system with an accessible non-return valve in the line.
Step 6 - Mounting the Vortex Filter
Mount the self-cleaning vortex filter (URVF01) vertically with adequate fall near the downpipe or beside the unit. Connect per the supplied specification.
Step 7 - Plumbing the Submersible Pump
Fit the supplied pump fittings and lifting chain, connect the discharge line to the outlet, and connect any mains changeover device if required.
Step 8 - Wiring the Pump and Pressure Switch
Mount the controller and pressure switch, run the cable in conduit, and have a licensed electrician complete all wiring. No electrical work is to be completed by an unlicensed person.
Step 9 - Inspection and Testing
Inspect and pressure test the system for leaks. Schedule periodic cleaning and function checks of filters and internal components going forward.
Step 10 - Landscaping and Commissioning
Complete paving or landscaping over the top. Ensure the lid is sealed before commissioning the system.
Polymaster underground rainwater systems must be installed by a qualified plumber or licensed installer. Installation by a non-licensed person may void the warranty. All electrical wiring must be performed by a licensed electrician. Follow the Polymaster Underground Water Tank Installation Guide to ensure a successful outcome.
DELIVERY ZONES
Delivery Across Victoria, South Australia, New South Wales, and Queensland
Preparing Your Site
To ensure a smooth arrival, please have the following ready before your order comes. Truck and trailer clearance required is 19 metres long, 5 metres high and 3.5 metres wide – check for low branches or powerlines on your access route. Ensure the gate and entrance can accommodate a 19m truck and trailer, and have a flat, clear area of at least 5 metres by 5 metres available for unloading. Our logistics team will contact you approximately seven days before arrival to confirm your address and run through the access requirements.
UNDERGROUND STORMWATER TANKS
Underground Stormwater Tanks and Rainwater Harvesting
Polymaster’s below-ground range is used across residential, civil and commercial projects for both rainwater harvesting and stormwater management. Whether you are collecting roof runoff to reduce your mains water bill, meeting a BASIX or council water storage requirement, or managing stormwater on a development site, a buried poly system is one of the most cost-effective and low-maintenance solutions available.
Going underground means your stored water stays cool, is protected from UV degradation, and is free from the algae growth that can affect above-ground options exposed to sunlight. Everything sits completely out of sight once commissioned, which means no impact on the visual amenity of your property – a key consideration for architects, landscape designers and residential developers working on tight urban blocks.
WATER TANKS CASE STUDIES
Trusted by Australian Businesses to Deliver These Projects
Polymaster’s underground water tanks are manufactured to the highest quality standards, independently certified, and trusted by homeowners, farmers, builders, and industrial operators across Australia.
Can't Find the Tank You Are Looking For?
Talk to our team. We will help you match the right round tank size, profile, and configuration to your property, water storage requirements, and budget.
MORE WATER TANK SOLUTIONS FROM POLYMASTER
Explore Our Full Range of Rainwater Tank Solutions
Our below-ground range is just one part of what Polymaster offers. If you need above-ground storage, high-capacity retention, or a solution for a specific site condition, we have you covered.
Round Water Tanks
Available from 200L to 31,700L, Polymaster’s round poly water tanks are the most versatile above-ground storage solution for rural, residential, and industrial applications across Australia.
View Range
Slimline Water Tanks
For narrow side yards, under-deck spaces and tight urban blocks, our slimline range delivers serious capacity without taking up serious space.
View Range
Retention and Detention Tanks
For civil, stormwater management, and OSD applications, Polymaster’s retention and detention range is engineered to council and development specifications.
View Range
FAQs
Frequently Asked Questions
Everything you need to know about Polymaster’s below-ground range, from sizing and setup through to maintenance and delivery.
Are underground water tanks good?
For most residential and urban applications, yes – going underground with your water storage offers a number of genuine advantages over above-ground options. Because the system is buried, it has no visual impact on your property, takes up no usable yard space, and the soil insulation keeps stored water cooler for longer, which reduces the risk of bacterial growth. The system is also protected from UV degradation and vandalism. The main trade-off is the excavation required during setup, which adds to the upfront cost. For properties where space, aesthetics or council regulations are a factor, a below-ground system is often the best solution available.
Are underground water tanks better than above-ground tanks?
It depends on your situation. Going underground is the better choice when space is limited, aesthetics matter, or you need to go under a driveway, lawn, or paved area. Above-ground options are generally easier and cheaper to set up, and easier to access for maintenance. For urban blocks, new developments, and properties where the system needs to be hidden, below-ground is typically the preferred choice for homeowners, architects, and builders.
Can I bury a 5,000L water tank underground?
Yes, provided you use a unit that is purpose-built and engineered for below-ground use. Polymaster’s 5,000L model (URT5000B and variants) is fully tested for ground and water pressure at depth, engineered using Finite Element Analysis, and certified to AS/NZS 4766. Standard above-ground poly tanks must never be buried – they are not designed to handle soil pressure or vehicle loads, and doing so will damage the unit and void the warranty.
What size underground water tank do I need for a 3-bedroom house?
For a typical 3-bedroom home, a 3,300L system is a practical starting point for garden and toilet use, while a 5,000L unit will give you greater capacity for household use or extended periods between rainfall events. The right choice depends on your roof catchment area, your average rainfall, and what you intend to use the stored water for. Use Polymaster’s online water calculator for a more accurate estimate, or call our team, and we can help you size the right system for your property.
What is the standard size of an underground water tank?
Polymaster’s below-ground range comes in two standard capacities – 3,300L and 5,000L – each available with or without a vortex filter and submersible pump. These sizes have been selected to suit the most common residential and light commercial requirements across Australia. Both share the same 1,640mm width and 1,665mm height, with the 5,000L model being 1,050mm longer.
Where should an underground water tank be placed?
Polymaster’s range can go under a lawn, garden bed, driveway, paved area, or entertaining deck – anywhere that provides the required excavation depth and access for setup. The best location balances proximity to your downpipes, access for the setup crew and equipment, and the end use of the water. If you are going under a driveway or trafficked surface, ensure you select the vortex filter option rated for vehicle loads. Your plumber or licensed installer can advise on the most suitable location for your specific property.
What are the disadvantages of underground water tanks?
The main considerations are the upfront cost, which is higher than above-ground options due to excavation and backfilling requirements, and the need for a qualified plumber and licensed electrician to complete the work. Access for inspection and cleaning requires entry via the riser, rather than the easy top access of an above-ground unit. For most urban residential buyers, these trade-offs are well worth the space savings and aesthetic benefits – but it is worth understanding the full scope of the project before committing.
Are underground water tanks safe?
Yes. Polymaster’s range is manufactured from food-grade polyethylene certified to AS/NZS 4020:2018 for products in contact with drinking water. The sealed, tamper-proof lid prevents unauthorised access and contamination. When set up by a qualified plumber in accordance with the Polymaster installation guide and regularly maintained, the system is a safe, reliable, and long-lasting water storage solution. Our tanks have a 15-year design service life and carry a 10-year manufacturer’s warranty when correctly installed.
How do underground water tanks work?
Polymaster’s poly systems store rainwater collected from your roof. Water flows from the gutters through your downpipes into the vortex filter, which removes debris and sediment before it enters the unit. Once stored, water is delivered to your taps, garden, or toilets either by gravity or via a submersible electric pump housed inside. The system includes an inlet, an overflow line connected to the stormwater drain to prevent flooding, and an access riser with lid for inspection and maintenance. Going below ground keeps the water cool, protects it from UV and maintains water quality between uses.
How do you install an underground water tank?
The process involves 10 key stages: excavation and base preparation, tank placement and levelling, pre-filling, backfilling in compacted layers, plumbing the inlet and overflow, mounting the vortex filter, plumbing and wiring the submersible pump, pressure testing, and final landscaping and commissioning. See the full step-by-step overview on this page, or download the Polymaster Underground Water Tank Installation Guide. All work must be completed by a qualified plumber and licensed electrician.
How do I pump water from an underground tank?
Polymaster systems include options for a submersible pump that sits inside the unit and delivers water under pressure to your home, garden, or both. A mains water changeover device can be added so your system automatically switches to the town supply when the stored water runs low. Our 80LPM submersible garden pump delivers up to 82 litres per minute with a fully automatic pressure system, run-dry protection, in-built electronic controls, and automatic reset.
How often should I clean an underground water tank?
We recommend periodic checks via the access riser – clearing the leaf trap and filter, and flushing any settled sediment from the base. As a general guide, inspect the system at least once a year and after any major storm events. If you have a vortex filter, the self-cleaning design significantly reduces the amount of sediment reaching your stored water. For confined space entry requirements, engage a licensed tradesperson. Follow Polymaster’s maintenance guidance for specific intervals.
How long do underground water tanks last?
Polymaster’s below-ground range has a 15-year design service life and carries a 10-year manufacturer’s warranty when set up correctly by a qualified plumber in accordance with the Polymaster installation guide. With proper maintenance, many units perform well beyond their design life.
Can slimline water tanks go underground?
No. Slimline tanks are designed for above-ground use only and must never be buried. They are not engineered to handle soil pressure, vehicle loads, or the structural demands of a below-ground setup. Doing so will damage the unit and void the warranty. Always use purpose-built models like Polymaster’s URT range, which are specifically engineered, FEA tested, and certified for below-ground use.