Mergansers are waterfowl with narrow, serrated beaks. Fossil evidence shows that the southern merganser (Mergus australis) was once common around coasts and in inland waterways. It was extinct on mainland New Zealand by 1500 AD, soon after the arrival of humans.

The species survived in the subantarctic Auckland Islands, but the last sighting was in 1902. By 1910, when the first reserve was set up there, the southern merganser was extinct, primarily because of predation by introduced animals.