Short answer: your tank’s overflow needs to go somewhere that carries the water safely away from the tank base and your house — usually into the stormwater system, a designated rubble pit, or a landscaped area that can absorb it without pooling or erosion. What it should never do is dump straight onto the ground next to the tank.
It’s one of those details that gets overlooked because it only matters once the tank is already full — but getting it wrong causes some of the most common (and preventable) problems we see on Australian properties. Here’s how to get it right.
Why overflow placement matters
Every tank overflows eventually. Even in a dry climate, a heavy storm after a long spell will fill a tank faster than you’d expect. When that happens, the water has to go somewhere — and if there’s no pipework directing it away, it runs straight down the side of the tank and pools at the base.
That causes two problems. First, erosion: repeated overflow events wash away the crusher dust or sand base your tank is sitting on, which can eventually undermine the tank’s stability. Second, it’s often just wasted water — a properly routed overflow can be put to use instead of soaking into the ground next to your house.
Your main overflow options
Stormwater connection
The most common setup for suburban and town properties. A pipe runs from the tank’s overflow fitting to the legal point of discharge on your stormwater system. This is usually the neatest, lowest-maintenance option, and it’s often a compliance requirement in built-up areas.
Rubble pit
Where there’s no stormwater connection available — common on rural and semi-rural properties — a rubble pit gives the overflow somewhere to drain into the ground gradually, away from the tank base and any structures.
Landscaped absorption area
Directing overflow into garden beds or absorbent landscaping can work well, provided the area can actually handle the volume without pooling, and it’s angled away from your house foundations, sheds, and any neighbouring property boundaries.
A second linked tank
If your tank overflows regularly, that’s often a sign it’s undersized for your roof catchment or household demand. Linking a second tank so the overflow feeds straight into it turns wasted overflow water into stored capacity — worth considering if you’re topping up during dry spells while also spilling water during wet ones.
What to avoid
Piping overflow directly onto bare ground next to the tank causes erosion and can undermine your base over time. Overflow should never be plumbed into a sanitary sewer line, and it shouldn’t be aimed toward a neighbour’s property, a shed foundation, or anywhere it could pool against a structure. If in doubt, pipe it at least a couple of metres away and downhill from the tank and any buildings.
Mosquito screening — a compliance essential
Every overflow point needs mosquito-proof mesh fitted, in line with Queensland Health Regulation and equivalent standards elsewhere. Without it, the overflow becomes an open door for mosquitoes to get into the tank and breed. It’s a small component, but a missing or damaged overflow screen is one of the most common ways pests end up in a tank system. Check it as part of your regular tank maintenance.
When frequent overflow means something’s wrong
An overflow doing its job occasionally after heavy rain is normal. An overflow running constantly, or every time it rains, usually means one of two things: your tank is undersized for the roof catchment feeding it, or your household usage isn’t keeping pace with what you’re collecting. Either way, it’s worth a conversation about right-sizing your system — sometimes the fix is a bigger tank, sometimes it’s a second linked tank, and sometimes it’s simply adjusting how the water’s being used.
Sort It Before the Next Downpour
None of this takes much to get right, and all of it costs more to fix after the fact — a washed-out base, a mosquito problem, or a tank that’s quietly outgrown the job it’s doing. Overflow is the one part of a tank system that only proves itself under pressure, so it’s worth sorting before the season turns, not after the first big storm shows you where the gaps are. If you’re not sure your setup’s up to it, our team’s happy to talk it through — call 1300 826 532.
Frequently Asked Questions
Where should a water tank overflow be piped to?
Ideally to your stormwater system, a designated rubble pit, or a landscaped area that can absorb the volume — always routed away from the tank base and any structures.
Can I just let the overflow run onto the ground?
No. Unpiped overflow causes erosion around the tank base and can undermine its stability over time. It should always be piped away.
Does my water tank overflow need mosquito screening?
Yes. Mosquito-proof mesh on every overflow point is a standard requirement and prevents mosquitoes breeding in the tank.
Why does my tank overflow so often?
Frequent overflow usually means the tank is undersized for your roof catchment or usage pattern. Linking a second tank or moving to a larger tank are the two most common fixes.
Can I link a second tank to catch overflow water?
Yes — overflow from the first tank feeds into the inlet of the second, giving you extra stored capacity instead of losing that water to the ground.