Notoreas ischnocyma was described by Edward Meyrick in 1905 using material collected at Castle Hill by George Hudson at an elevation of 1700 meters.

[1][2] Hudson had previously illustrated the specimen in his 1898 book New Zealand Moths and Butterflies (Macro-lepidoptera) but had included it within the species N.

Fore-wings triangular, costa slightly arched, termen obliquely rounded; dark fuscous, with some scattered yellow-whitish scales; subbasal, first, median,second, and subterminal lines slender, whitish, subbasal straight, first nearly straight, somewhat irregular, median indistinct, curved outwards in disc, second waved throughout, angulated in middle, indented beneath middle, subterminal irregularly waved: cilia white, basal half barred with dark fuscous.

[8][9] Other than the type locality of Castle Hill, N. ischnocyma has also been found in the Hawkdun and the Dansey ecological districts in Otago.

[5][11] Larvae of species within the genus Notoreas feed exclusively on plants within the genera Pimelea and Kelleria.