Although adult specimens of I. steropastis are relatively easy to recognise they might possibly be confused with I. inscripta, I. theobroma or with darker forms of I. arotis.

However I. steropastis can be distinguished as it has a long dark basal forewing streak that these three species lack.

This species was first described by Edward Meyrick in 1887 as Mamestra steropastis using specimens collected in Napier, Blenheim and Christchurch by R.W.

[2][4] George Hudson described the larva of this species as follows: About 1+1⁄2 inches in length, of almost uniform thickness, considerably flattened; the head is ochreous, the body very pale ochreous-brown; there are no distinct markings on the thoracic segments, except a few minute black dots round the middle of each; the rest of the body is covered with a number of very fine blackish lines, which become darker posteriorly, and are stronger on the dorsal and lateral regions; there is a row of minute black dots round the middle of each segment; the spiracles are black and the underside of the larva is faintly tinged with green.

[4] The larvae of I. steropastis can be distinguished from I. arotis as there are differences in the placement of the P1 head capsule and the anal shield has a minute double mound between the D2 setae which is lacking in I.

Forewings moderately dilated, costa hardly arched, apex obtuse, hindmargin crenulate, obliquely rounded; reddish-fuscous; veins dark fuscous, margined with whitish or ochreous-whitish; a short black median streak from base; a slender dark reddish-fuscous longitudinal streak in disc from above apex of this to 3⁄5, posterior extremity somewhat dilated; a minute white discal dot near beyond its extremity; a very oblique short blackish streak from inner margin near base; second line hardly indicated; subterminal hardly traceable except by two very long whitish dentations touching hind-margin below middle : cilia reddish-fuscous, slenderly barred with whitish.

[4] Although I. steropastis is relatively easily recognisable it might possibly be confused with I. inscripta, I. theobroma or with darker forms of I.

[4] In particular Austroderia fulvida, A. richardii and C. selloana are all recorded as larval host species of I. steropastis as is Poa foliosa.

[7] The larvae of this species is regarded as a pest as it chews distinctive notches in the sides of the flax leaves.