How to use these dimensions to plan your install
The three measurements that drive every install decision are length, width and height. Length and width set the footprint of the excavation; height sets the depth. Inlet pipe gradient from the dwelling (typically 1:60 minimum for waste lines) determines how deep the tank's inlet has to sit, which in turn sets total excavation depth.
Add 200–300mm on every side of the tank for sand bedding and backfill. Add 300–600mm above the tank lid for cover. For a typical 1500L install, the hole works out at roughly 2,500 × 1,600 × 1,500mm. For a 2500L, plan for approximately 3,200 × 1,700 × 1,600mm. Your licensed plumber will finalise based on the inlet gradient and any site-specific constraints.
Why dimensions matter for site access
On constrained sites — narrow side passages, established gardens, rear-yard granny flats — the tank's width is the dimension that decides whether it can be carried in by hand or has to be craned over the dwelling. At 1,110mm wide and 61kg, the 1500L tank fits through any standard gateway and can be tipped on its side to pass narrower openings. The 2500L at 1,210mm wide is also gate-friendly on most lots.
This is one of the main practical reasons builders prefer poly over concrete on tight sites. A 600–2,000kg concrete tank with the same capacity needs a HIAB truck and clear overhead lifting access — neither of which a typical suburban granny flat site has.
Frequently asked questions
What are the dimensions of a 1500L septic tank?
The Bush Billabongs 1500L polyethylene septic tank measures 2,030mm long × 1,110mm wide × 1,120mm high. It weighs 61kg empty, so it can be delivered on a standard ute and lifted into position by two people without machinery.
What are the dimensions of a 2500L septic tank?
The Bush Billabongs 2500L polyethylene three-chamber septic tank measures 2,750mm long × 1,210mm wide × 1,180mm high. Empty weight is approximately 110kg.
How deep does a septic tank need to be buried?
Cover depth typically ranges 300–600mm above the tank lid, with access risers brought up to ground level for inspection and pump-out. The minimum excavation depth equals the tank height plus the cover depth plus a sand bedding layer — roughly 1.5–1.8m for a 1500L tank and 1.6–1.9m for a 2500L. Your licensed plumber confirms based on inlet pipe gradient and site conditions.
How big a hole do I need to dig for a septic tank?
Allow at least 200–300mm of working clearance on all sides for backfill and bedding. For a 1500L tank, an excavation of approximately 2,500 × 1,600 × 1,500mm is typical. For a 2500L, allow approximately 3,200 × 1,700 × 1,600mm. Final dimensions depend on plumber preference and site access.
Can a 1500L septic tank fit through a standard gate?
Yes. At 1,110mm wide, the 1500L tank fits comfortably through any standard 1.2m gateway, and it can be tipped on its side to pass through narrower openings. Two people can carry the empty 61kg tank by hand for short distances.
Do I need a crane to install a poly septic tank?
No. Poly septic tanks are light enough to lower into a prepared excavation using ratchet straps, an excavator bucket or basic lifting gear. Concrete tanks at the same capacity require a HIAB truck or crane because they weigh 600–2,000kg.