Shell small, 9 mm, heavy, medium spire with axis of coiling slightly offset. Protoconch globose, large, 2 whorls, 0.53 mm high, 0.53 mm wide. Teleoconch 7 whorls, pentagonal with 5 flattened ribs/whorl, continuous on succeeding whorls. Ribs highest just below midpoint of whorl, become shallower then have secondary peaks. Sutures wavy, highest on ribs. Numerous fine growth lines. Body whorl slightly flattened, last half lacks ribs. Columella straight, sinus deep with callus, broadens at end. Aperture thin, cylindrical. Anterior siphonal canal broad but deep, outer lip thickened. Shell glossy off-white colour, aperture white. Length 9 mm, width 4.1 mm.

Three syntypes of Plagiostropha opalus (Reeve, 1845) were examined in the BM(N H), registration number 19631069. The two species are very close, but differ in that P. opalus is slimmer, the body whorl is not as angular, and P. opalus has up to 35 fine spiral striae on the whorls which are absent on P.flexus. These do not reach the anterior end of the body whorl of P. opalus. The two are treated here as separate species.

Type locality: 124 m depth, 160 km north of Croker I., Northern Territory. Known only from the type locality.