The Auckland Island merganser (Mergus australis) (Māori: Miuweka),^[3] also known as the New Zealand merganser, was a typical merganser that is now extinct.

This duck was similar in size to the red-breasted merganser (Mergus serrator). The adult male had a dark reddish-brown head, crest and neck, with bluish black mantle and tail and slate grey wings.^[4] The female was slightly smaller with a shorter crest.

The Auckland Island merganser was known from the Auckland Islands archipelago, part of the New Zealand Subantarctic Islands. The only historical records are from Auckland Island and Adams Island. Holocene bones were found on Enderby Island.^[5] The last specimens were apparently a pair shot on 9 January 1902, and the species is now extinct.^[5] It was not found in a 1909 search, and a thorough 1972/1973 exploration of possible habitat concluded that it was long extinct.^[6] Its decline was caused by a combination of hunting and predation by introduced mammals.^[7]

The closely related Chatham Island merganser (Mergus milleneri) is known from Holocene bones found on Chatham Island, New Zealand.^[8] The species identity of merganser bones from mainland New Zealand - North, South, and Stewart Islands - is unresolved.^[5]