I. bromias was first described by Edward Meyrick in 1902 using four specimens collected from Chatham Island.

[3] The lectotype specimen is held at the Natural History Museum, London.

Head and thorax grey, sometimes whitish-mixed, variably tinged with brownish or reddish, and indistinctly marked irregularly with blackish streaks.

Forewings moderately dilated, costa almost straight, apex obtuse, termen crenulate, obliquely rounded; grey, puitially whitish-sprinkled, variably and irregularly tinged with purple-brownish, median band and terminal area darker; a short black median longitudinal streak from base; a short similar parallel streak from dorsum near base, sometimes obsolete; first and second lines slender, blackish, waved, nearly obsolete on upper half, fairly distinct and rather nearly approximated on lower half; spots outlined with black, more or less rosy-brownish, especially reniform, orbicular and reniform sometimes whitish-edged, orbicular semi-oval, incomplete above, claviform subtriangular; subterminal line white, sometimes interrupted or partially obsolete, with one prominent dentation below middle (on vein 3), near dorsum more conspicuous and preceded by a triangular dark fuscous spot, elsewhere edged with smaller dark fuscous spots : cilia fuscous, mixed with dark fuscous, and indistinctly barred with whitish.

[1] Adult female I. bromias release pheromones to attract males at the lower temperature of 15 degrees Celsius compared to other species in the Ichneutica genus.