Thousands of birds have been found dead at one of Western Australia's most important inland wetlands.

Birds roosting at Lake Gregory after a wet season. Credit:File photo/DCBA

A Department of Primary Industry and Regional Development veterinarian arrived at Lake Gregory in the East Kimberley earlier this month to take sample, and found a number of wild birds in "poor conditions with low body weights".

On a department inspection, it was found several thousand birds had died at the wetland, but the cause of death remains a mystery.

Lake Gregory is a permanent freshwater lake located between the Great Sandy Desert and the Tanami Desert and was previously described by the department as the most important inland wetland in Australia.