An adult Willie wagtail is between 19 and 21.5 cm (7.5 and 8.5 in) in length and weighs 17–24 g (0.6–0.85 oz), with a tail 10–11 cm (approx 4 in) long. The short, slender bill measures 1.64–1.93 cm (around 0.75 in),[9] and is tipped with a small hook.This species has longer legs than other fantails, which may be an adaptation to foraging on the ground.[The male and female have similar plumage; the head, throat, upper breast, wings, upperparts, and tail are all black, with a white eyebrow, "whiskers" and underparts. The bill and legs are black and the iris dark brown. Immature birds in their first year after moulting from juvenile plumage may have pale tips in their wings,[9] while juvenile birds themselves have duller plumage, their upperparts brown-tinged with some pale brown scallops on the head and breast.

The wagtail is very "chatty" and has a number of distinct vocalisations. Its most-recognised sound is its alarm call which is a rapid chit-chit-chit-chit, although it has more melodious sounds in its repertoire. The alarm call is sounded to warn off potential rivals and threats from its territory and also seems to serve as a signal to its mate when a potential threat is in the area. John Gould reported that it sounded like a child's rattle or "small cog-wheels of a steam mill".