Bellbirds are green with a short, curved bill, slightly forked tail, and noisy whirring, fast and direct flight. Adult males are olive green, slightly paler on the underparts, with a head tinted purple; wings and tail blackish. Female are browner with narrow white-yellow stripe across the cheek from the base of the bill, and bluish gloss on top of head. Adults of both sexes have wine-red eyes. Juveniles are similar to females, but with yellowish cheek stripe, brown eyes and lacking the bluish gloss on the head.

Voice: song varies regionally but is ringing notes without grunts or wheezes. Alarm call a rapidly repeated harsh “yeng,”. The famous bell notes noted by Sir Joseph Banks in Queen Charlotte Sound on James Cook first voyage to New Zealand are only heard when many birds are present at once, mostly at dawn and dusk in places of high bellbird density with few other bird species.

Similar species: greenfinch is smaller with a large, pale, conical bill and no purple or blue on the head. Tui is very different in appearance but has similar song that can be distinguished (usually) by the inclusion of additional less musical notes than bellbird, e.g. grunts, clicks and wheezes.