Kadua haupuensis is a shrub in the Rubiaceae (coffee) family from 1 to 1.5 m (3.2 to 5 ft) tall. Stems are brittle with terete (circular in cross-section) branchlets, and glaborus (without hairs) except for minute hairs around the nodes. Older branches are brown with longitudinaly fissured bark. Leaf shape is oblong-lanceolate to oblong-ovate, 3 to 11.5 cm (1.1 to 4.5 in) and 0.7 to 3.2 cm (0.3 to 1.3 in) wide. There are three to six secondary vein pairs which are translucent when fresh. The petioles (the stalk that joins a leaf to a stem) are 2 to 16 m (0.08 to 0.6 in) long and narrowly winged and the stipules (outgrowths borne on either side of the base of the petiole) are externally glabrous with sheaths 1.5 to 3 mm (0.06 to 0.1 in) long. The corolla lobe length of staminate flowers is 2.5 to 3.5 mm (0.1 to 0.14 in) and pistillate (unisexual and is 6 to11 mm (0.2 to 0.4 in) long in pistillate (female) flowers. There are 10 to 67 flowers per inflorescence, flowers with calyx, (sepals of a flower, typically forming a whorl that encloses the petals and forms a protective layer around a flower in bud), 4-lobed, lobes are linear- oblong to linear-oblanceolate (significantly longer than wide and widest above the middle, gradually widening toward the apex). Seed capsules are broadly obovoid (egg-shaped), 3.5 to 4.5 x 4 to 5 mm (0.1 x 0.16 x 0.2 in) and glabrous. Seeds are numerous, dark brown or blackish, 0.5 to 0.6 x 0.3 to 0.4 mm (0.02 to 0.02 x 0.01 to 0.02 in), irregularly obovate (egg shaped with the narrower end at the base) to ellipsoid (sphere-like), compressed laterally and the surface is pimple-like with prominent bubble-like protuberances.