Pronotum (greenish-) grey with yellow margins, elytra dark red with yellow margins, and a short yellow strip below the scutellum. Similar in colouration but otherwise very different to T. flavocincta in its narrower body shape, frons and underside much less hairy, and pronotum more finely and evenly punctate.

The type locality of this species is given as Adelaide, and I have examined 14 specimens from seven sites in the Adelaide area, one of which is illustrated here. These all appear relatively slender in form and there is good evidence for an association with River Red Gum Eucalyptus camaldulensis and the prospect of it being their larval host. Recently, three observations of T. flavomarginata made during the summer of 2021-2022 were posted on the iNaturalist website; all were from the Adelaide region and in areas where River Red Gums occur.

Superficially similar beetles that were previously grouped under this name (although flagged as a potentially distinct taxon) are now treated on this website as T. cf. marginalis (q.v.). While resembling T. flavomarginata in colouration, they are thicker in form and approach T. marginalis in appearance. As they were collected in mallee areas remote from E. camaldulensis they presumably have a different host.

An unnamed species mentioned and distinguished by Tepper 1887 under Stigmodera flavocincta may actually be T. flavomarginata. He notes that 'it appears now to be very rare' and refers to a specimen he picked up in the street at Kent Town (an inner city suburb of Adelaide).

There is circumstantial evidence for breeding in River Red Gum Eucalyptus camaldulensis and for the adults feeding on its flowers which appear in December and January. The reference by Tepper 1887 to what might be this species frequenting 'the flowers of the White Gum' is puzzling but most likely refers to white-barked forms of E. camaldulensis; the only other native white-barked gum around Adelaide is E. leucoxylon which can be discounted because it flowers there in April and May.