How Underground Water Tanks Work and Why More Australians Are Installing Them

Rainwater storage does not have to occupy a large section of the backyard. Underground Water Tanks store collected roof runoff below ground, leaving more surface space available for gardens, entertaining areas, pathways and other uses.

For Australian properties where outdoor space is limited or landscaping is a priority, placing water tanks underground can provide a practical alternative to large above-ground systems. The tank itself is only one component. Roof catchments, gutters, screens, first-flush devices, pumps, plumbing and overflow arrangements all affect how effectively collected rainwater can be stored and reused.

Understanding how these systems work helps homeowners choose an appropriate capacity, location and tank configuration before excavation begins.

How Do Underground Water Tanks Collect Rainwater?

Rainwater harvesting begins on the roof.

Water flows from the roof into gutters and downpipes before being directed towards the tank. Screens and strainers can intercept leaves and larger debris before they enter the storage system.

A first-flush device provides another layer of protection. NSW Health recommends these devices because they divert the first portion of runoff after a dry period, reducing the amount of accumulated dust, bird droppings, leaves and other debris washed from the roof into the tank.

Once screened, the water enters the underground tank and remains stored until required.

Because the tank sits below the level of most taps and appliances, a pump is generally needed to move rainwater through the property's plumbing. When rainfall fills the tank beyond its capacity, an appropriately designed overflow directs excess water into the property's approved stormwater arrangement.

The system therefore operates through five main stages: collection, screening, storage, pumping and overflow management.

Why Put the Tank Underground?

The most obvious advantage is space.

An above-ground tank permanently occupies part of a garden, driveway side passage or area beside the house. On smaller suburban blocks, finding enough surface space for several thousand litres of storage can be difficult.

An underground tank places most of that volume below the surface, allowing the area above or around it to remain visually less dominated by water infrastructure.

This can be particularly useful when a homeowner wants to preserve:

●        Garden and lawn areas

●        Entertaining spaces

●        Narrow side access

●        Views from the house

●        Existing landscaping

●        Space around pools or outdoor structures

Underground poly tanks are also available in different shapes. Current Australian ranges include low-profile designs with shallower installation depths and vertical tanks that use a smaller footprint but require deeper excavation.

The right shape depends on the site's available footprint, excavation depth and surrounding structures.

How Large Should an Underground Tank Be?

Bigger is not automatically better. Tank capacity should reflect how much water the property can collect and how regularly that water will be used.

The Australian Government's YourHome guidance recommends considering:

●        The intended uses for rainwater

●        Expected household demand

●        Local rainfall patterns

●        Roof area connected to the tank

●        Desired security of supply

A useful rule for estimating collection potential is that 1 mm of rainfall falling on 1 square metre of roof produces approximately 1 litre of rainwater before collection losses are considered.

For example, a 100 m² roof receiving 20 mm of rainfall has a theoretical catchment of about 2,000 litres.

Actual collection will be lower because some rainfall is lost through first-flush diversion, evaporation, gutter overflow and other factors.

Regular demand also matters. A tank used for toilets, laundry and garden watering creates storage capacity between rainfall events. A large tank that is rarely used may remain full and provide little additional capture during the next storm.

What Can Stored Rainwater Be Used For?

Rainwater systems are commonly designed around non-drinking household and outdoor uses.

NSW planning guidance lists supported uses including toilet flushing, clothes washing, garden watering, car washing, hot-water systems and filling swimming pools, outdoor spas and ornamental ponds.

The intended uses should be decided before installation because they influence tank size, pump requirements and plumbing.

A system supplying only garden taps may be relatively simple. Connecting rainwater to toilets or washing machines requires additional plumbing and appropriate separation from the mains water supply.

Where rainwater and mains systems interact, relevant plumbing and backflow-prevention requirements need to be addressed by appropriately qualified trades.

Site Conditions Matter Before Excavation Begins

Installing water tanks underground requires more planning than placing a freestanding tank beside a building.

Before excavation, the site should be assessed for:

●        Available machinery access

●        Existing pipes and underground services

●        Distance from buildings and retaining structures

●        Required excavation depth

●        Soil and drainage conditions

●        Groundwater

●        Proposed landscaping above the tank

●        Potential vehicle or structural loads

Groundwater deserves particular attention because an empty or partially empty underground tank can be subjected to substantial upward buoyancy forces.

Installation guidance for some polyethylene underground tanks specifically requires additional anti-buoyancy measures where groundwater conditions are present. It also advises that installation requirements should be considered alongside site-specific engineering advice.

This is why an ordinary above-ground polyethylene tank should never simply be buried. The tank must be specifically designed for underground installation and installed according to its manufacturer's requirements.

Can You Drive or Build Over an Underground Tank?

Not automatically.

The fact that a tank is buried does not mean the surface above it can support cars, driveways, sheds or other structures.

Some underground tanks are available with trafficable lid options, but the tank, lid, surrounding installation and load conditions all need to be appropriate for the intended use.

If the area will experience vehicle loads or support another structure, this requirement should be identified before the tank is ordered.

Trying to change a pedestrian installation into a trafficable one after excavation and landscaping have been completed can be difficult and expensive.

Check Planning Requirements Before Installation

Planning requirements differ between states, councils and individual properties.

In NSW, below-ground rainwater tanks may qualify as exempt development only when all relevant development standards are satisfied. Current NSW Planning Portal guidance places specific restrictions on where below-ground tanks can use this pathway, including requirements relating to land zoning, acid sulfate soils and environmentally sensitive areas.

The guidance also states that works must be structurally adequate, installed according to the manufacturer's specifications and comply with applicable building requirements.

Homeowners should therefore check the requirements applying to their particular property rather than assuming every underground tank can be installed without approval.

This is particularly important when excavation is close to boundaries, buildings, heritage features or significant vegetation.

Maintenance Starts Above the Ground

Putting the tank underground does not remove the need for maintenance.

NSW Health describes rainwater harvesting as “low maintenance, not no maintenance”. It recommends regularly checking the roof catchment, gutters, screens and tank components to prevent contamination and mosquito entry.

A practical routine should include:

●        Clearing leaves and debris from gutters

●        Checking inlet screens and strainers

●        Cleaning first-flush devices

●        Inspecting overflow screens

●        Checking pump operation

●        Looking for damaged plumbing connections

●        Monitoring accessible areas for sediment or contamination

NSW Health advises inspecting and clearing roof catchments and gutters approximately every three to four months and checking mosquito-proof screens regularly.

Access for maintenance should therefore be included in the original installation plan. Tank lids, filters, pumps and other serviceable components should not become inaccessible after paving or landscaping is completed.

Why Underground Storage Can Make Sense

Water tanks underground offer a practical solution when homeowners want meaningful rainwater storage without sacrificing a large area of usable outdoor space.

Their effectiveness depends on much more than capacity. Roof catchment area, rainfall, household demand, tank shape, excavation conditions, groundwater, plumbing and maintenance all influence how well the system performs.

A properly planned system collects roof runoff, removes larger contaminants, stores the water below ground and uses a pump to supply suitable household or outdoor uses.

For properties where space is valuable, underground storage can make rainwater harvesting easier to integrate into the overall design of the home. Matching the tank to both the site and expected water use helps create a system that remains practical long after the excavation has been filled and the landscaping completed.