Kadua haupuensis is a subdioecious shrub, standing 1–1.5 meters tall, with erect, brittle stems. The branchlets are terete (cylindrical), displaying a glabrous surface except for minutely puberulent nodes, and mature branches showcase brown, longitudinally fissured bark. The leaves are simple and opposite. They have oblong-lanceolate to oblong-ovate blades ranging from 3 to 11.5 cm long and 0.7 to 3.2 cm in width. The upper surfaces are glabrous and lustrous, while the lower surfaces may be glabrous or sparsely hirtellous, especially along the costa, and are chartaceous. The secondary veins are festooned brochidodromous (venation in which the secondary veins do not terminate at the margin but join to form a series of prominent arches) with three to six pairs, and the tertiary veins are conspicuously reticulate and weakly prominulous on both surfaces, appearing translucent when fresh. Petioles range from 2 to 16 mm long, are narrowly winged, and exhibit a glabrous or puberulent abaxial side. The stipules are externally glabrous, forming a sheath 1.5–3 mm long with an awn 2–5 mm long. The flowers are organized in (6–)10–67-flowered, cymose-paniculiform inflorescences, measuring 2–4 × 2–4.5 cm, and are subtended by a pair of leaf-like bracts. The flower structures are either glabrous or glabrate, with primary branches in two to three pairs and monochasial ultimate branches bearing three to twelve flowers each, subtended by glabrous or sparsely puberulent bracts. The hypanthium is obconic and glabrous. These flowers are unisexual, with occasional hermaphroditic flowers on staminate plants. Calyx lobes are linear-oblong to linear-oblanceolate, 2.5–6 mm long, and carry a green with purple tint when fresh. The corolla of staminate flowers is tubular, ranging from 10–16 mm in length, while pistillate flowers have a shorter corolla tube. Stamens in staminate flowers are polleniferous, whereas in pistillate flowers, anthers are abortive and non-polleniferous. The fruit is a broadly obovoid capsule, measuring 3.5–4.5 × 4–5 mm, glabrous, weakly bisulcate, and weakly compressed laterally. Seeds are numerous, dark brown or blackish, irregularly obovate to ellipsoid, and compressed laterally with a papillose surface featuring prominent bubble-like protuberances. The seeds contain well-developed oily endosperm, although the endosperm may occasionally be scanty or absent. It is found only in north-facing mesic forests at an elevation of approximately 366 meters.