Coprosma kauensis is a dioecious shrub or tree 2-6 mm tall. The young stems of this plant are characterized by their strigose to coarsely puberulent surface or, in rare cases, appressed hirtellous texture. The leaves are simple and arranged oppositely on the stem. Leaf blades come in shapes ranging from elliptic-oblanceolate to oblong-elliptic or spatulate, measuring 42–120 mm long and 15–35 mm wide. The apex of the leaves can vary from acute to rounded, with an attenuate base. The upper surfaces of the leaves are primarily glabrate or sparsely strigose, while the lower surfaces are prominently strigose, especially along the veins, contributing to the leaves’ chartaceous quality. The margins of the leaves are entire, with a pinnate venation pattern. Petioles measure 6–20 mm in length, and the stipules, which are 5–10 mm long, are connate in the lower quarter to half, featuring strigose surfaces and occasionally a glabrous patch on the upper half. The flowers are typically grouped in 3(-5) clusters, with 6-9-merous flowers on either glabrous or densely coarsely puberulent peduncles. The plant is dioecious, distinguishing between male and female flowers that are often heterostylous and pollinated by insects. The calyx in staminate and pistillate flowers is cup-shaped with irregularly whitish ciliate teeth. The corolla varies between the sexes; in staminate flowers, it is campanulate, whereas in pistillate flowers, it is narrowly funnelform. The reproductive parts include stamens that alternate with the corolla lobes and an inferior ovary that is occasionally partly so, or rarely superior, with 2(-4) cells. The fruit is drupaceous, juicy, and bright reddish-orange, measuring 7–9 mm long. The seeds are contained within 2(-4) one-seeded, planoconvex pyrenes and possess a well-developed oily endosperm, although in some cases, the endosperm may be scanty or absent. This species primarily inhabits mesic to wet forests and is found at elevations ranging from 600 to 1,330 meters.