Austromitra koumacensis: Shell tiny to 5.0 mm, fusiform-elongate. Protoconch with 1.5 white glassy whorls, mamillate in shape. Shell smooth and shiny, spire outline straight to slightly concave. Teleoconch with 3.5-4.5 whorls, whorl outline slightly convex, suture well defined and incised, whorls sculptured with 12-15 rounded, raised axial ribs with equally sized interspaces in between; on the last quarter of the body whorl the axial ribs become reduced and are nearly obsolete towards the outer lip. Aperture narrow, light brownish with a faint white band in the centre, outer lip rounded, smooth, thin, weak lirations visible within the aperture in most specimens, columella with 4 folds, brownish, apex fold largest, folds gradually getting smaller towards the base, 3-4 fine spiral cords emerging from the columellar folds present on the siphonal fasciole. Siphonal canal narrow, shell thin. Base colour white, shoulder light brown, fine interrupted dark-brown dashes between the axial ribs at the base of the white zone at the periphery of all whorls, light brown anterior half of the body whorl, getting lighter towards the base.

Austromitra koumacensis: Austromitra koumacensis n. sp. is comparable to A. cernohorskyi TURNER, 2008 from South New Caledonia because of the white shell with brown ornaments, but the new species is smaller (~ 5 vs. ~ 8 mm) and the spire looks broader because the single whorls have nearly the same diameters at posterior and anterior end, whereas A. cernohorskyi is characterized by rain drop-like whorls with a smaller diameter at the posterior end. Further on the brown markings in A. koumacensis n. sp. are different. There is a brown band below the suture present on all whorls whereas the fine brown line in A. cernohorskyi is not present on the first two whorls. There is an additional fine brown line in the centre of the white zone on all whorls. And there is not only a narrow brown peripheral band on the body whorl, but the anterior half of this whorl is completely light brown coloured in A. koumacensis n. sp.
The new species is also similar to A. valarieae LUSSI, 2015 (Figs 25-26) from South Africa because of size and brown colouration, but A. valarieae has a more angulated shoulder, fine spiral striae in the anterior half, which are missing in the new species and the protoconch is always brown instead of white. Further on the white zone can be also light brown which is not known from A. koumacensis n. sp.

Austromitra koumacensis: Type Locality
New Caledonia, Koumac area, east of ile Balabic, 20°40'S, 164°15'E, 26-40 m.
Known only from the area of Koumac (New Caledonia) at a depth of 40 to 90 m.