How rainwater tanks actually work

Every tank sits at the end of a chain. Rain lands on a roof, sheets toward gutters, concentrates into downpipes, and is directed either to the tank inlet or to stormwater. The roof is the real collector; the tank only keeps what the roof delivers. A clean, well-sized catchment with tidy gutters will out-perform a large tank fed by a small or leaky roof.

Because inflow arrives in bursts — a heavy hour can deliver more than a dry fortnight — storage exists to smooth supply across dry spells. The art is matching the buffer to the rhythm of local rainfall rather than to a catalogue size.

The first water off a roof carries the most dust, leaf matter and droppings. A first-flush diverter sets aside that initial volume so it never enters the tank — a simple idea that does most of the work of keeping stored water clean. Beyond it, an inlet screen blocks leaves and excludes mosquitoes.

At the other end, every tank needs a planned overflow. Once full, a tank simply passes surplus through its overflow outlet, ideally routed away from foundations to a garden, soak or stormwater connection. Overflow is not a fault; it is the design working. The questions that matter are how often it happens and where the water goes.