Mandora Marsh in north-western Western Australia floods at intervals of several years to many decades. It flooded in 1999 and 2000, supporting 480 000 ± 40 000 and 490 000 ± 50 000 waterbirds in August of each year. Counts of Black-winged Stilts (~180 000 and 220 000) represented 70% and 80%, respectively, of the estimated Australian population, and counts of Hardheads (~100 000) and terns (~110 000) comprised considerable portions of the Australian populations of these species. Breeding was prolific and the Marsh appears to be an important breeding site, on the rare occasions it floods, for many species. Waterbird numbers, breeding and movements are discussed in the context that, collectively, the network of arid-zone wetlands, including Mandora Marsh, has a major role in maintenance of Australian waterbird populations. The significance of flooding history and grazing on the biological value of arid-zone wetlands are also discussed briefly.