Gahnia vitiensis subsp. kauaiensis is a tall, perennial sedge characterized by woody, erect to spreading rhizomes. The culms are terete (cylindrical), smooth, and reach a height of 120–160 cm, with diameters of 5–10 mm near the base. They are 4–6-noded between the base and the inflorescence. The leaves are simple, numerous, and both basal and cauline. Blades are linear and coriaceous, typically as long as or longer than the culm, measuring 10–15 mm in width. The leaf apex is gradually tapering and ends in a long caudate-acuminate tip. Margins are involute and serrulate–scabrous. Veins are parallel. Basal sheaths are dark purplish brown or dusky brown, while cauline sheaths are green with a purplish tinge toward the base. Stipules are absent. Inflorescences are large, effuse, and decompound panicles, measuring 30–90 cm in length and 5–15 cm in width. These are densely branched and bear numerous spikelets, sometimes interrupted in the lower section. Leafy bracts are narrowly linear or subsetaceous and are usually slightly shorter than the inflorescence, with dark purplish brown sheathing bases 2–5 cm long. Peduncles are produced in groups of 2–4 and are branched; bracteoles are black and spathaceous. Spikelets are solitary, obovoid in fruit, 3–3.5 mm long and about 1.2–1.5 mm wide, each containing a single flower. There are 4–5 glumes per spikelet: the lower 1–2 are dark brown, membranous, ovate, 2.5–3 mm long with acuminate, short-awned apices, and are one-nerved; the upper 3 are blackish brown, ovate, and about 2 mm long with obtuse or mucronate tips. Flowers may be bisexual or staminate. Both calyx and corolla are absent. There are 3 stamens with filaments around 10 mm long, often strongly flexuous or crisped. The ovary is superior with elongate styles; the base of the style is conical-subulate and remains persistent at the apex of the achene. The fruits are trigonous achenes, obovoid–ellipsoid, 1.7–2.6 mm long and 1–1.5 mm wide, glossy, and yellowish-brown at first, turning blackish-brown with maturity. Each achene contains a single seed with a thin seed coat free from the pericarp, a basal embryo, and abundant endosperm. It is found in wet forests and along bog margins, primarily within the vicinity of Alaka‘i Swamp to Mount Wai‘ale‘ale on Kaua‘i, between 610–1,590 meters.