Cyanea kauaulaensis is an unarmed shrub that grows 2–4 meters high and is extensively branched from the base, producing many basal shoots. The stems are light brown, erect to arching, and can extend up to 6 meters long. They often lean on adjacent vegetation and roots where they contact the soil. The leaf scars on the stems are subcircular to broadly obovate–depressed, with white latex. The leaves are simple and clustered near the ends of the branches. Adult plants have elliptic, oblong, or elliptic–oblong blades measuring 19–30 cm in length and 5–7 cm in width, with an attenuate, acuminate, or cuspidate apex and an often asymmetrical, attenuate to cuneate base. The leaves are light green when fresh, moderately glossy on the upper surface, and dry to a membranaceous to chartaceous texture. It bears axillary inflorescences with up to 30 flowers per stem, which are mainly perpendicular to the stem and have 5–20 flowers each. The corolla is white, tubular, and gently curved to suberect, with the tube being 23–27 mm long and the linear–subulate lobes reflexed. The stamens are connate with a glabrous staminal column and anthers 6–7 mm long. The fruits are bright orange when ripe, globose to obovoid, and 8–10 mm in diameter, with a smooth surface crowned by an apicular ring. The seeds are numerous, ellipsoid, and embedded in translucent pulp, with a shiny brown seed coat adorned with faint striations, approximately 0.6-0.7 mm long. It occurs in riparian sites and basalt boulder-strewn slopes along perennial streams within a Metrosideros lowland wet forest at 732-914 meters.