Diagnosis: Shell conical, spire profile straight to slightly concave, base slightly concave; 2 rows of 8-17 nodules on last whorl, peripheral nodules crossed by 2-3 threads, threads on base sometimes nodulose; projecting anterior lip, aperture trapezoid; grey or black with white to cream nodules. Penial filament tapering, half total length of penis; copulatory bursa divided, ventral branch equal to or longer than dorsal branch
Shell: Mature shell height 5.0-15.4 mm. Shape conical to high-conical (H/B = 1.31-1.69; SH = 1.62-2.17); spire whorls lightly rounded, suture not distinct; spire profile straight to slightly concave; periphery of last whorl angled or rounded; profile of base slightly concave. Columella concave, hollowed at base, flared to form projecting anterior lip; apertural outline trapezoid; usually a small eroded parietal and pseudumbilical area. Sculpture of last whorl: 2 rows of rounded to pointed nodules, at periphery and shoulder, axially aligned in 8-17 pairs; entire surface (including base) with 14-24 strong narrow spiral threads and microstriae, peripheral nodules crossed by 2-3 major threads; basal threads (below peripheral nodules) 5-8, sometimes bearing small nodules. Protoconch 0.26 mm diameter, 2.6 whorls. Colour: black to dark brown (fading to blue-grey), paler at suture and on base, nodules white to cream; aperture dark brown with pale band at base; columella dark brown.

See remarks on E. malaccana.
In eastern Australia the distribution off. austrotrochoides and the conchologically similar N. pyramidalis hardly overlap, the latter being found only at and south of the Keppel Islands, and there are only two records of sympatric occurrence (Reid & Williams 2004). The distinction between the E. malaccana group (as E. trochoides) and N. pyramidalis was described by Reid & Williams (2004). Comparing only the two species in Australia: the shell of N. pyramidalis reaches 27 mm (cf. 15.4 mm in E. austrotrochoides), is broader (H/B = 1.23-1.46, cf. 1.31-1.69), has a single row of nodules on the penultimate whorl (usually 2 visible in E. aus¬trotrochoides), basal ribs are never nodulose (sometimes nodulose in E. austrotrochoides), and there are two pale bands within the dark aperture (cf. a single band). The papillate penial filament of male N. pyramidalis and additional loop of the capsule gland in females are diagnostic anatomical differences.
There is some variation in the shells of E. austrotrochoides; as in other members of the E. malaccana group shells from limestone are tall and show strong sculpture.

Tropical Australia, eastern New Guinea, New Caledonia.

Habitat and ecology: This species is common in the upper littoral fringe. It occurs on basalt, granite, limestone, sandstone and concrete, in sheltered to moderately exposed sites in continental settings. On the Queensland mainland it occupies the highest levels of the littoral fringe, above the zone of E. melanacme and E. vidua (Endean et al. 1956a), but on the offshore islands it extends to a lower level, around mean high water or high water of neap tides (Endean et al. 1956b; W. Stephenson et al. 1958). It was described as a typical mainland species, of sparse occurrence on islands such as Heron Island (Endean et al. 1956b). Black & Johnson (2001) described zonation and demography at Ningaloo, Western Australia; this species occurred at the highest level on the shore and showed high survival and slow growth, taking nearly 3 years to reach half maximum size.