Head, pronotum and elytra predominantly dark metallic blue; pronotum with yellow margins and elytra with three transverse yellow bands; legs and ventral side metallic green.

This species was named by Hope 1843 in honour of a Mr Fortnum who collected the type material in Adelaide. Hope 1845 specifically notes; 'The above insect was taken at the new settlement named Adelaide'. Tepper 1887 describes it as 'a rare species, being extremely shy and swift flying'.

Temognatha fortnumii has a definite association with Casuarina trees. Goudie 1920 reported a specimen taken at Birchip in western Victoria 'flying amongst a grove of bull-oak trees' [= Allocasuarina luehmannii]. The specimen mapped here from the MU region was collected at Koomooloo, ENE of Burra, from 'Black oak scrub' [= Casuarina pauper]. Tepper 1887 established Drooping Sheoak Allocasuarina verticillata (then known as Casuarina quadrivalvis) as a larval host on the basis of adult specimens that he extracted from its roots and trunks.