A large, distinctive duck with a somewhat goose-like shape, most often found around coastal wetlands, tidal mudflats, and nearby grassy fields. From a distance it can appear simply black and white, but up close the head shows a deep glossy green and the broad chest band is a rich rusty colour. The male has a noticeable knob at the base of the red bill, while juveniles are duller with a pale face and eye-ring. Inland it is scarcer and more local, occurring on lakes and rivers. It is frequently seen walking and grazing, sometimes in groups.