These are bulky geese and their almost uniformly grey plumage, bearing rounded black spots, is unique. The tail and flight feathers are blackish and the legs are pink with black feet. The short, decurved black bill and green cere gives it a very peculiar expression.

The Cape Barren Goose is 75 to 100 cm (30–40 in) long, weighs 3 to 7 kg (7–15 lb) and has a 150 to 190 cm (60–75 in) wingspan; males are somewhat larger than females. This bird feeds by grazing and rarely swims. Their ability to drink salt or brackish water allows numbers of geese to remain on offshore islands all year round. They are one of the rarest of the world's geese. [4] It is gregarious outside the breeding season, when it wanders more widely, forming small flocks. It is a most peculiar goose of uncertain affiliations (Sraml et al. 1996). It may either belong into the "true geese" and swan subfamily Anserinae or into the shelduck subfamily Tadorninae as distinct tribe Cereopsini, or be separated, possibly including the prehistorically extinct flightless New Zealand Geese of the genus Cnemiornis, in a distinct subfamily Cereopsinae. Indeed, the first bones of the New Zealand birds to be discovered were similar enough to those of the Cape Barren Goose to erroneously refer to them as "New Zealand Cape Barren Goose" ("Cereopsis" novaezeelandiae).