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.

[5] It was not found in a 1909 search, and a thorough 1972/1973 exploration of possible habitat concluded that it was long extinct.

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