The Australasian bittern (Botaurus poiciloptilus) is a large and stocky waterbird with a thick neck and camouflaged plumage. The upperparts of the Australasian bittern are dark brown to black, with mottled buff patterns to aid in camouflage. Its underside is pale yellow to buff with streaks of brown, while its chin and throat are pale. This species has a thick, black stripe on the side of the neck and a pale stripe above the eye.

The Australasian bittern has a buff-coloured bill which is straight and pointed. Its legs and feet are pale green to olive, and its eyes are orange-brown to yellow. All bitterns have long legs and toes which are useful for wading in swamps and wetlands.

Variation in the colouration of the Australasian bittern has been observed, with individuals showing darker or paler plumage, but this is not well understood. The male and female Australasian bittern are similar in appearance, but the female is smaller than the male. Juveniles are similar in appearance to the adult, but are paler and have a rufous-streaked underside and heavy buff flecking on the back.

The Australasian bittern has a characteristic booming call which can be heard during the mating season. A short, harsh ‘craak’ is given in alarm.