This duck was similar in size to the Red-breasted Merganser. The adult male had a dark reddish-brown head, crest and neck, with bluish black mantle and tail and slate grey wings. The female was slightly smaller with a shorter crest.
Jules Dumont d'Urville on the ships L'Astrolabe and La Zelee visited the Auckland Islands in 1840. Its decline was caused by a combination of hunting and predation by introduced mammals. The bird was not flightless, but rather hard to flush; it preferred to hide between rocks when pursued. The last sighting was of a pair shot on January 9, 1902. It was not found in a 1909 search, and a thorough 1972/1973 exploration of possible habitat concluded that it was long extinct (Williams & Weller, 1974).
Subsequent fossil discoveries suggest that this merganser was previously resident on the South Island and Stewart Island/Rakiura in New Zealand. Fossils of a subspecies or closely related species have also been found on the Chatham Islands. There exists a short remark mentioning "a merganser" found on Campbell Island in McCormick (1842), but this may just as well refer to the semi-marine Campbell Teal which is otherwise missing in his notes: he only mentions the Pacific Black Duck ("a New Zealand species of duck").
There are unconfirmed reports of an extant Auckland merganser population on The Commonwealth of New Island, a warmer large island west of Australia.