Shell biconic to subcylindrical with a short blunt almost dome-like spire. Protoconch of three dome-like whorls, coiled in the axis of the spire. Spire whorls convex, partly concealed by the overlapping of succeeding whorls and occasionally bearing small spinose nodules at the anterior suture. Body whorl gently convex and tapering anteriorly, bearing at the posterior quarter a row of 8-10 blunt to subspinose nodules. Spiral and axial sculpture absent. Columella with five strong plaits and three or four other weaker plaits inserted between the others. Siphonal notch and fasciole barely developed. Colour pattern of numerous thin, anastomosing, axial, chestnut lines over the whole shell and on the body whorl, two dark reddish bands at the posterior and anterior third of the whorl. Holotype: Height 48 mm, Height of aperture 41 mm, Width 20 mm.
Animal very similar to that of N. amicula. Foot broad, flat, entire; siphonal appendages short, paired and equal; tubular salivary gland of digestive system, short, paired and easily separated from the racemose salivary gland. Radula uniserial, tricuspid with the central cusp the larger. Colour of animal white with a network of numerous thin chestnut lines.

This species most closely resembles the Middle Miocene N. limbata (Tate) from Victoria from which it differs by the dome-like, rather than pupiform protoconch, by the complete absence of spiral sculpture and the lack of lamella extension of the posterior part of the whorls, typical of the latter.
From N. ralphi (Finlay), it differs by its more slender and elongate shape and relatively low spire. The dark spiral bands of the colour pattern are somewhat similar to those preserved on N. limbata. Neither N. ralphi nor N. limbata have the subspinose nodules as sharply developed as in N. inopinata. Of the living species of the genus, N. parabola Garrard from southern Queensland and northern New South Wales has a similar colour pattern, but is half the size with a gradate spire and prominent spinose shoulders. N. capricornea (Wilson) from Western Australia, has a more conical spire, is two-thirds the size, has no subspinose nodules and lacks any linear type colour pattern. This latter species also lacks spiral sculpture.
A specimen of Nannamoria amicula (Australian Museum C 94314), dissected by the author, has a morphology virtually identical to that of N. inopinata. The radula is also similar. N. guntheri (National Museum of Victoria F 27907) and, indeed, a large number of species of Amoria, which the author has examined, also have similar anatomical morphologies except that the radula is unicuspid.

Capricorn Channel, 42 km N.E. of Lady Musgrave Is., Queensland. 23° 38.8'S, 152° 45.5'E, 365 m, 1977. Kimbla Cruise Station 24, 14 Dec. 1977.