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.

[5] The last specimens were apparently a pair shot on 9 January 1902, and the species is now extinct.

[6] Its decline was caused by a combination of hunting and predation by introduced mammals.

[8] The species identity of merganser bones from mainland New Zealand - North, South, and Stewart Islands - is unresolved.