Gahnia lacera is a naturalized perennial with short, woody rhizomes with tufted culms at 1.5-3 meters tall, 2-4 mm in diameter in the lower part, subterete and smooth. The leaves are numerous, primarily basal, a few cauline, with coriaceous (leathery) blades, linear, as long as or longer than the culm at 6-9 mm wide. The margins are involute (that curl toward the top of the leaf). The sheaths are elongated, basal sheaths are pale to dark brown, and the cauline ones are dark green and tinged with dark brown. The inflorescence is an erect, compound panicle 15-50 cm long, 2-3 cm wide, bearing 5-7 partial panicles. The lower leafy bracts are longer than the inflorescence, the upper ones gradually shorter but usually longer than the subtended partial panicle, the sheaths 3-9 cm long, tinged with pale or dark brown, peduncles 2-3 together, the lower ones up to 8 cm long, the upper ones shorter, 1-2 times further branched, bracteoles brown, somewhat spathaceous, ca. 1 cm long. Spikelets are ellipsoid-turbinate at 7 mm long and 2-3 mm wide. There are four glumes that are brown to reddish brown, the lower two empty and longer than the upper 2, lanceolate at 6.5-7 mm long, acute, the midnerve prolonged into an erect awn 1.5-2 mm long, the upper two glumes subtending the achene, ovate to ovate-elliptic, apex obtuse; perianth absent; stamens 4; filaments persistent; style filiform, 12 mm long, deeply 3-cleft ca. 2/3 its length. The fruit is a purplish-black achene, fusiform, 3.5-4(-4.5) mm long, and 1.5-2.0 mm wide, acute at both ends. It occurs at 915-1065 meters in elevation.