The adult male South Island robin is dark grey-black over the head, neck, mantle and upper chest; the flight feathers and tail are brownish-black, and the lower chest and belly white to yellowish white with a sharp demarcation between black and white on chest. Adult females are light to dark grey over the upper body. They further differ from males in the  white chest-belly area being smaller and not having such a distinct demarcation between grey and white feathering. Juveniles are similar to females, but often with a smaller or no white patch on the underparts. Adults of both sexes are able to expose a small white spot of feathers above the base of the beak during intraspecific and interspecific interactions.

Voice: South Island robins have four recognisable vocalisations. Fullsong is a series of phrases given loudly by males only, generally from a high perch. Robins can be heard giving fullsong year round, but particularly during the breeding season. It is used to indicate territorial occupancy and to attract a mate – bachelors spend much more time singing than paired males. Subsong is similar to fullsong but given at much less volume, is given by both sexes, and most frequently during the moult. The downscale is a series of very loud ‘chuck’ calls, descending in tone, and which start in rapid succession and finish slowly. The call lasts 3-4 seconds, is given by both sexes, and is most frequently heard during the non-breeding season (January-June). The fourth vocalisation type is the ‘chuck’, which is given as single notes (contact calls) or in rapid succession and loudly (as an alarm call) when a predator is nearby.

Similar species: there are no species that are similar to the robin in the South Island or Stewart Island. Robins are much larger and lack the white wing-bars of tomtits.