Tūī have almost-black heads, underparts, wings and tails that have an iridescent blue and green sheen, especially on the head and wings. The upper back and flanks are dark reddish brown with a bronze sheen, the nape and sides of the neck have filamentous white feathers, and there are two unusual curled white feather tufts on the throat (poi). Small white shoulder patches on the upperwing show prominently in flight, but are usually concealed when perched. The bill and feet are black, and the eye dark brown. The sexes are alike, but the male is larger. Adults have a notch on the 8th primary, and this feather quivers from the narrow part creating the whirring sound in flight. At first, juveniles have a browner body and lack the throat tuft, but after a few months they are only distinguishable in the hand from the lack of notch on the 8th primary.

Tūī are notoriously aggressive, and will defend a flowering or fruiting tree, or a small part of a large tree, from all-comers, whether another tui or another bird species. They vigorously chase other birds away from their feeding territory with loud whirring wings. Tūī have a display flight, in which they fly upwards above the canopy, and then make a noisy, near-vertical, dive back into the canopy. Tūī play a very important role in the dynamics of New Zealand forests because they are one of the most common pollinators of flowering plants, and also disperse the seeds of trees with medium-sized fruits.

Tūī diet varies depending on the seasonal availability of nectar and fruits. Their preferred diet is nectar and honeydew, and they will often shift to, or commute daily or more frequently to, good nectar sources, such as stands of puriri, kowhai, fuchsia, rewarewa, flax, rata, pohutukawa, gums and banksias. In the breeding season, tūī supplement their nectar diet with large invertebrates such as cicadas and stick insects obtained by hawking or by gleaning from the outside of trees. In the autumn, medium-sized fruit such as wineberry, kaikomako, mahoe, ngaio, rimu or kahikatea, make up much of the diet. In winter, flowering gums, banksias, puriri, and tree lucerne are important nectar sources, along with sugar-water feeders in gardens.

Eggs are laid from September to January. The nest, built by the female, is a rough bulky structure of twigs and sticks, lined with fine grasses, high in the canopy or subcanopy. The clutch is 2-4 white or pale pink eggs, marked with reddish-brown spots and blotches. Incubation and brooding is by the female only. Chicks are initially fed only by the female, but later the male helps to feed them.

Tūī are widespread and locally abundant on the North, South and Stewart Islands, and their offshore islands; they are scarce only in drier, largely open country east of the Southern Alps. Tūī are present on the Kermadec and Auckland Islands, and there is a larger subspecies endemic to the Chatham Islands. Tūī are absent on the Poor Knights Islands probably due to the very high density of bellbirds there competing for a limited nectar resource. Tūī are found in native forest and scrub (sometimes in exotic forests), and in rural gardens, stands of flowering kowhai and gums, and in suburban parks and gardens. There is much local movement, when tūī follow a seasonal succession of flowering or fruiting plants. They usually nest in native forest and scrub, but will commute more than 10 km daily to feed on rich sources of nectar.

Tūī declined over their entire range, becoming near extinct in highly modified areas. In areas with no pest control they are infrequent. Tūī are not considered threatened with extinction as they are more resilient to pests than other deeply endemic birds and have recovered in many areas where pest control is present. The Chatham island subspecies is threatened.

Within the Brook Waimārama Sanctuary’s pest-free forest, tūī are thriving. The abundance of flowering plants and honeydew-producing insects provides ideal feeding conditions, while predator control supports higher nesting success. Visitors regularly hear tūī song echoing through the valley — a clear sign of a healthy, functioning forest ecosystem.

Tūī are among the family of songbirds known as honeyeaters (Meliphagidae) spread across Australasia and the Pacific. They are equipped with long, brush-tipped tongues capable of efficiently collecting nectar from most flowers.