The smallest owl on the Australian mainland, the Australian boobook is 27 to 36 cm (10.5 to 14 in) long.
The nominate subspecies is the largest.
It has short, rounded wings and a short tail, with a compact silhouette in flight.
Australian boobooks on the Australian mainland follow Bergmann's rule, in that birds from cooler and more southerly parts of the range tend to be larger.
Thus, birds from the Canberra region weigh around 300 g (11 oz) while those from the Cape York Peninsula and Broome are around 200 g (7.1 oz).
Females tend to be a little larger and heavier than males, with males weighing 146–360 g (5.1–12.7 oz) and females 170–298 g (6.0–10.5 oz).

The Australian boobook has generally dark brown head and upperparts, with white markings on the scapulars and spots on the wings.
Its head lacks tufts common in other owls, and has a paler facial disk, with a white supercilium (eyebrow) and dark brown ear coverts and cheeks.
The brown feathers of the upper forehead, above the supercilium, and sides of the neck have yellow-brown highlights.
The feathers of the lores, chin and throat are white with black shafts.
The feathers of the underparts are mostly brown with white spots and dark blue-grey bases.
The upper tail is dark brown with lighter brown bars and a grey fringe at the end, while the undertail is a lighter grey-brown.
The female tends to be more prominently streaked than the male overall, though this is inconsistent and wide variation is seen.
The eyes have been described as grey-green, green-yellow, or even light hazel.
The bill is black with a pale blue-grey base and cere.
The feet are greyish to pinkish brown with dark grey to blackish claws.
The underparts are pale, ranging from buff to cream, and are streaked with brown.
The overall colour is variable and does not appear to correspond to subspecies or region.
In northern and central Australia, Mayr found that the colour of the plumage appears to correlate with the rainfall or humidity, paler birds being found in three disjunct areas, each around 1,600 kilometres (990 mi) away from the other two: the western Kimberley and Pilbara, Sedan on the Cloncurry River, and around Ooldea, with darker birds found on Cape York and Melville Island.

Young Australian boobooks are usually paler than adults, and do not attain adult plumage properly until their third or fourth year.
Juveniles (up to a year old) have whitish underparts and foreneck, a larger and more prominent pale eyebrow and larger whitish spots on their upperparts.
The tips of their feathers are white and fluffy, remnants of the nestlings' down.
These are worn away over time, persisting longest on the head.
The feathers of the head, neck and underparts are fluffier overall.
Immatures in their second and third year have plumage more like adults, though their crowns are paler and more heavily streaked.
The smallest owl on the Australian mainland, the Australian boobook is 27 to 36 cm (10.5 to 14 in) long. The nominate subspecies is the largest. It has short, rounded wings and a short tail, with a compact silhouette in flight. Australian boobooks on the Australian mainland follow Bergmann's rule, in that birds from cooler and more southerly parts of the range tend to be larger. Thus, birds from the Canberra region weigh around 300 g (11 oz) while those from the Cape York Peninsula and Broome are around 200 g (7.1 oz). Females tend to be a little larger and heavier than males, with males weighing 146–360 g (5.1–12.7 oz) and females 170–298 g (6.0–10.5 oz).
The Australian boobook has generally dark brown head and upperparts, with white markings on the scapulars and spots on the wings. Its head lacks tufts common in other owls, and has a paler facial disk, with a white supercilium (eyebrow) and dark brown ear coverts and cheeks. The brown feathers of the upper forehead, above the supercilium, and sides of the neck have yellow-brown highlights. The feathers of the lores, chin and throat are white with black shafts. The feathers of the underparts are mostly brown with white spots and dark blue-grey bases. The upper tail is dark brown with lighter brown bars and a grey fringe at the end, while the undertail is a lighter grey-brown. The female tends to be more prominently streaked than the male overall, though this is inconsistent and wide variation is seen. The eyes have been described as grey-green, green-yellow, or even light hazel. The bill is black with a pale blue-grey base and cere. The feet are greyish to pinkish brown with dark grey to blackish claws. The underparts are pale, ranging from buff to cream, and are streaked with brown. The overall colour is variable and does not appear to correspond to subspecies or region. In northern and central Australia, Mayr found that the colour of the plumage appears to correlate with the rainfall or humidity, paler birds being found in three disjunct areas, each around 1,600 kilometres (990 mi) away from the other two: the western Kimberley and Pilbara, Sedan on the Cloncurry River, and around Ooldea, with darker birds found on Cape York and Melville Island.
Young Australian boobooks are usually paler than adults, and do not attain adult plumage properly until their third or fourth year. Juveniles (up to a year old) have whitish underparts and foreneck, a larger and more prominent pale eyebrow and larger whitish spots on their upperparts. The tips of their feathers are white and fluffy, remnants of the nestlings' down. These are worn away over time, persisting longest on the head. The feathers of the head, neck and underparts are fluffier overall. Immatures in their second and third year have plumage more like adults, though their crowns are paler and more heavily streaked.
Australian boobook