T. agrionata was first described by Francis Walker in 1862 using specimens collected either in Hawkes Bay or Taupō by William Colenso.

[3] George Vernon Hudson discussed and illustrated this species in his 1928 book The Butterflies and Moths of New Zealand.

On the forewings the green colouring is more or less confined to the neighbourhood of the veins and is much less pronounced than in T. tipulata; the basal and subterminal areas are transversed by several narrow cream-coloured bands, which are especially evident in the male, and the black markings are more extensive and slightly tinged with dull reddish.

[7] This species can be distinguished T. tipulata as it lacks the Z-shaped basal line and has a conspicuous large pale patch near tornus.

Robert Hoare has also stated that T. agrionata has is a very distinct black V on the forewing dorsum near the base that appears to be a distinguishing feature as it is not present on T.

[7] T. agrionata frequents forest habitat and can be found up to an elevation of 900 m.[7][8] The host species for the larvae of this species are native leafy Loranthaceae such as Alepis flavida, Ileostylus micranthus, Peraxilla colenso, Peraxilla tetrapetala, Trilepidea adamsii and Tupeia antarctica.