The Blue-billed Duck is one of only two Australian species of stiff-tailed ducks - diving ducks with spine-like tail-feathers. It is a small and compact duck, with a length of 40 cm. The male's head and neck are glossy black, and the back and wings are a rich, chestnut to dark-brown. During the summer breeding season the male's bill turns bright blue. The female is brownish-black above, with narrow bands of light brown and mottled light brown and black below. The female's bill is dark grey-green. In the non-breeding season the male resembles a dark female. The tail is usually held flat on the water, although during courtship, or when alarmed, it is held fully erect.