The little shag is a small shag that exhibits bewildering plumage variation. Most adults are black with white cheeks and throat, a colour morph sometimes referred to as a ‘white-throated shag’. At the other extreme, 8-60% of adults (depending on location) have completely white underparts, from face to undertail, indistinguishable from the little pied shags/cormorants of Australia. ‘Intermediate morph’ or ‘smudgy’ are terms used for about 5% of birds that have white faces and throats and variegated patches of black and white on the breast and belly. All three morphs develop a crest during the breeding season. The bill is short, stout and yellow, dark on the ridge. The eye is brown, and facial skin yellow. Legs and feet are black. The pied morph of the little shag is more common in the north of New Zealand.

In 1987, white-breasted birds made up 60% of the population in the Far North, 32% in Auckland, 15% in the rest of the North Island, but just 8% in the South Island, where the white-throated morph is predominant. The first plumage of juveniles is either white-breasted or entirely black. Examples of each type can be present in the same nest where the adults are of different plumage forms. The all-black fledglings develop a white throat after becoming independent and until then can be confused with little black shags (but note differences in bill colour and shape).

Further variation is added by birds with stained white feathers, appearing orange-breasted. This may be due to iron-staining and persists for an extended period, for example throughout the nesting period of a recognised individual. All little shags have relatively longer tails than black, pied and little black shags. When flying or roosting with wings held out, their wings are also shorter and broader than these three other ‘black-footed’ shag species.

Voice: silent except at nesting colonies, where males give characteristic bi- or tri-syllabic ‘cooing’ sounds during courtship displays. A greeting call, consisting of a series of notes, uh, uh, uh, uh, fading away, is used by both sexes when approaching the nest to change over or to bring food.

Similar species: black shag, pied shag, and little pied shag all have plumage that is similar to one of the patterns seen in adult or juvenile little shag colour morphs. In all ages and plumages, little shags can be distinguished from these three other species by their small size combined with a short, stout yellow bill and relatively long tail.