This species is similar in appearance to P. sandycias but can be distinguished from it as P. plinthina has palpi that are longer and has more blurred forewing markings.

[3] The male holotype specimen was collected by A. Purdie in July in Wellington and it is held at the Natural History Museum, London.

[2] George Hudson in his 1898 book New Zealand moths and butterflies (Macro-lepidoptera) placed this species in the Chloroclystis genus.

Forewings with hindmargin very oblique, somewhat bowed, slightly sinuate above anal angle; whitish-ochreous, suffusedly irrorated with reddish, tending to form very obscure lines; a blackish irroration towards costa on basal fourth; edge of median band marked by series of black dots on veins, anterior from 3⁄8 of costa to 2⁄5 of inner margin, curved, posterior from before 3⁄4 of costa to 2⁄3 of inner margin, upper 2⁄3 tolerably regularly curved; a pale waved subterminal line preceded by a reddish fascia, forming a darker red spot above middle, another above anal angle, and a blackish costal spot; a black hindmarginal line : cilia ochreous-whitish, with an obscure reddish line, basal half with reddish-fuscous spots on veins.

[7][8][5] P. plinthina is on the wing mainly from June until September with occasional observations up to December, and as such it is regarded as a winter specialist.