Shell tiny to 6.0 mm, fusiform-elongate, protoconch with 1.5-2.0 dark brown glassy whorls, mamillate in shape. Shell smooth and shiny, spire outline undulate. Teleoconch with 4-4.5 whorls, whorl outline convex, suture well defined and incised, early whorls sculptured with 11-13 rounded, raised axial ribs with equally sized interspaces in between. Interspaces sculptured with 3-6 shallow, indistinct, spiral grooves which do not bisect the axial ribs. Body whorl with 14-15 rounded, raised axial ribs, interspaces sculptured with 7-8 very weak, indistinct, spiral grooves, axial ribs remain of uniform size on the aperture and dorsal side of the shell. On the last quarter of the body whorl the axial ribs terminate where 3-4 fine, round, oblique axial cords emerge from the aperture. Aperture narrow, outer lip flares out near attachment area on body whorl, gently rounded, smooth, thin, lirations are not visible within the aperture in any of the specimens we studied. Columella with 3 folds, apex fold largest, folds gradually getting smaller towards the base. Siphonal canal short and narrow, shell thin and semi-transparent on outer lip near siphonal canal. Base colour white, early whorls white with a faint interrupted brown dashed line between the axial ribs located just above the suture of each whorl, below the suture a subsutural yellow to light brown band is found, a wide white band is located

Austromitra willcoxae n. sp. is perhaps most closely related to Austromitra koumacensis n. sp. (Fig. 18, holotype). The two species are found in totally different faunal zones but share a similar shell outline. The protoconchs differ in shape, the one of A. willcoxae being more mamillate in outline, the shell more transparent, yellow-brown in overall colour and pattern.
We also compare this species to the South African fauna. Austromitra valarieae LUSSI, 2015 recently described is similar in size with a more bulbous protoconch and with a variable colour pattern from straw yellow to dark brown.

Type Locality
South Australia, Smoky Bay, -36 m (20 fathoms) depth.
Known only from South Australia between Smoky Bay and Port MacDonnell.