Which lake is the largest lake in Australia by surface area when it fills?

Kati Thanda–Lake Eyre is Australia’s largest lake by surface area when it fills.

It is an ephemeral salt lake in South Australia’s Lake Eyre Basin. The lake lies about 15 metres below sea level and usually consists of dry salt flats or shallow saline water, but exceptional floods can spread across roughly 9,500 square kilometres.

Its basin receives water from several inland river systems, including the Warburton River. Because evaporation is intense and inflows are irregular, complete filling is rare and the lake’s appearance changes dramatically.

The traditional name Kati Thanda reflects the lake’s significance to the Arabana people. It is often called simply Lake Eyre, after Edward John Eyre, but the dual name is the official form used in Australia.