The huia had black plumage with a green metallic tinge and distinctive rounded bright orange wattles at the gape.
In both sexes, the eyes were brown; the beak was ivory white, greyish at the base; the legs and feet were long and bluish grey while the claws were light brown.
Huia had twelve long glossy black tail feathers, each tipped for 2.5–3 cm (0.98–1.18 in) with a broad band of white.
Immature huia had small pale wattles, duller plumage flecked with brown, and a reddish-buff tinge to the white tips of the tail feathers.
The beak of the young female was only slightly curved.
Māori referred to certain huia as huia-ariki, "chiefly huia".
The huia-ariki had brownish plumage streaked with grey, and the feathers on the neck and head were darker.
This variant may have been a partial albino, or perhaps such birds were simply of great age.
Several true albino huia were recorded.
A white specimen painted by John Gerrard Keulemans around 1900 may have been the result of progressive greying or leucism, rather than albinism; the current whereabouts of this specimen are unknown.

Although sexual dimorphism in bill shape is found in other birds, such as the riflebirds, sicklebills and other wood-excavating birds including some species of woodpecker, it was most pronounced in the huia.
The beak of the male was short at approximately 60 mm (2.4 in) and slightly arched downwards and robust, very similar to that of the closely related saddleback, while the female's beak was finer, longer at around 104 mm (4.1 in), and decurved (curved downward) like that of a hummingbird or honeyeater.
The difference was not only in the bone; the rhamphotheca grew way past the end of the bony maxilla and mandible to produce a pliable implement able to deeply penetrate holes made by wood-boring beetle larvae.
The skulls and mandibles of the huia and saddleback are very similar, the latter essentially miniatures of the former.

There are two possible explanations for the evolution of this sexual difference in bill shape.
The most widely supported is that it allowed birds of different sexes to utilise different food sources.
This divergence may have arisen because of a lack of competitors in these foraging niches in the North Island forest ecosystems.
The other idea is that the ivory-coloured bill, which contrasted sharply with the bird's black plumage, may have been used to attract a mate.
In animals that use sexually dimorphic physical traits to attract a mate, the dimorphic feature is often brightly coloured or contrasts with the rest of the body, as with the huia.
It has been suggested that as the female was the main provider of food for the chicks by regurgitation, this sex evolved the longer bill to obtain the protein-rich invertebrate diet required for the chicks.

Another, less obvious aspect of the huia's sexual dimorphism was the minor size difference between the sexes.
Males were 45 cm (18 in) long, while females were larger at 48 cm (19 in).
Additionally, the tail of the male was about 20 cm (7.9 in) in length and the wingspan was between 21 and 22 cm (8.3 and 8.7 in), while the female's tail was 19.5 to 20 cm (7.7 to 7.9 in) and the female's wingspan was 20 to 20.5 cm (7.9 to 8.1 in).
The huia had black plumage with a green metallic tinge and distinctive rounded bright orange wattles at the gape. In both sexes, the eyes were brown; the beak was ivory white, greyish at the base; the legs and feet were long and bluish grey while the claws were light brown. Huia had twelve long glossy black tail feathers, each tipped for 2.5–3 cm (0.98–1.18 in) with a broad band of white. Immature huia had small pale wattles, duller plumage flecked with brown, and a reddish-buff tinge to the white tips of the tail feathers. The beak of the young female was only slightly curved. Māori referred to certain huia as huia-ariki, "chiefly huia". The huia-ariki had brownish plumage streaked with grey, and the feathers on the neck and head were darker. This variant may have been a partial albino, or perhaps such birds were simply of great age. Several true albino huia were recorded. A white specimen painted by John Gerrard Keulemans around 1900 may have been the result of progressive greying or leucism, rather than albinism; the current whereabouts of this specimen are unknown. 
Although sexual dimorphism in bill shape is found in other birds, such as the riflebirds, sicklebills and other wood-excavating birds including some species of woodpecker, it was most pronounced in the huia. The beak of the male was short at approximately 60 mm (2.4 in) and slightly arched downwards and robust, very similar to that of the closely related saddleback, while the female's beak was finer, longer at around 104 mm (4.1 in), and decurved (curved downward) like that of a hummingbird or honeyeater. The difference was not only in the bone; the rhamphotheca grew way past the end of the bony maxilla and mandible to produce a pliable implement able to deeply penetrate holes made by wood-boring beetle larvae. The skulls and mandibles of the huia and saddleback are very similar, the latter essentially miniatures of the former.
There are two possible explanations for the evolution of this sexual difference in bill shape. The most widely supported is that it allowed birds of different sexes to utilise different food sources. This divergence may have arisen because of a lack of competitors in these foraging niches in the North Island forest ecosystems. The other idea is that the ivory-coloured bill, which contrasted sharply with the bird's black plumage, may have been used to attract a mate. In animals that use sexually dimorphic physical traits to attract a mate, the dimorphic feature is often brightly coloured or contrasts with the rest of the body, as with the huia. It has been suggested that as the female was the main provider of food for the chicks by regurgitation, this sex evolved the longer bill to obtain the protein-rich invertebrate diet required for the chicks.
Another, less obvious aspect of the huia's sexual dimorphism was the minor size difference between the sexes. Males were 45 cm (18 in) long, while females were larger at 48 cm (19 in). Additionally, the tail of the male was about 20 cm (7.9 in) in length and the wingspan was between 21 and 22 cm (8.3 and 8.7 in), while the female's tail was 19.5 to 20 cm (7.7 to 7.9 in) and the female's wingspan was 20 to 20.5 cm (7.9 to 8.1 in).
Huia