Tree to 50 m or more in height and 150 cm dbh, often fluted and buttressed at base, the trunk clear of branches for a considerable height. Submature trees have a conical crown. Bark dark grey, covered irregularly with small protuberances ca. 1-2 mm across and 1-2 mm high, scaling off in large, ovoid flakes. Branchlets slender and drooping. The kahikatea seedling bears juvenile foliage until 1-2 m high, when semi-adult or adult foliage may appear. The juvenile, semi-adult and adult foliage stages often occur together on the same tree. Juvenile leaves are subdistichous, subpatent, narrow-linear, subfalcate, acuminate, decurrent, 3-7 × 0.5-1 mm. Adult leaves are 1-2 mm long, imbricate, appressed, keeled, subtrigonous, lanceolate-subulate to acuminate, with broader base. Pollen cones are terminal, up to 1 cm long, with bisporangiate sporophylls, apiculus acute. Seeds are solitary, terminal on short branchlets, upper 2-3 leaves forming a distinct receptacle, red, swollen and succulent when in fruit. A male tree glows a faint orange when the cones are mature, a female red when its berry-like receptacles are ripe. Seeds are 4-5 mm long, broadly ovoid, black, nutlike (Allan 1961, Salmon 1996, Metcalf 2002).

Most of the year, kahikatea does not bear recognizable reproductive structures, and in its native forest it is commonly so tall that foliage characters are of little use in identification. The bark characters and tree form are thus the principal aid in field identification. Also, it sheds foliage and branchlets fairly constantly, so fallen foliage is often available to confirm identification. The bark of young trees is relatively undistinctive, but that of mature trees is not readily confused with that of any other native New Zealand conifer except miro (Prumnopitys ferruginoides), which usually lacks buttressing and has highly distinctive foliage