Herb, to 1.2 m tall, sparsely hirtellous to glabrate; stems obtusely quadrangular, branched. Blades lanceolate to lanceolate-linear, acuminate at apex, gradually attenuate at base to an obscure, winged petiole, 5-16 cm long, 0.5-4 cm wide, entire; cystoliths dense but +/- obscure. Flowers sessile, in dense axillary clusters subtended by linear-lanceolate bracts; calyx 5-9 mm long, deeply lobed, the lobes +/- linear; corolla white, bilabiate, 6-8 mm long, puberulent, the tube ca 1 mm diam, the upper lip bidentate, ca 2.5 mm long, the lower lip trilobate; stamens 4, arranged in 2 unequal pairs and fused to tube at base; pistillate parts not seen. Capsules narrowly oblong, acute at apex, 9-12 mm long, ca 1.5 mm wide, glabrous; seeds 16-18, disk-shaped, less than 1 mm wide, brown, the margin discolorous and becoming sticky on wetting. Croat 6402. Frequent on sandbars in lagoons around the edge of the island, especially on the western side. Flowers from November to March, especially from December to February. Since the fruits develop quickly, the same inflorescence may bear both flowers and fruits.