I think there is confusion of this species with T. cosmodes, T. eremicola, T. isophragma, etc., the variability of each being comparable with their differences.
Looking at the images I can see on the ALA and the LBH the other species mentioned look distinctly and consistently diffferent to me, To my eye, T. lechrioleuca seems to be the only one of these similar Thallarcha species with a foreward pointing black 'v' mark or chevron on the white thorax.
I note BoldSystems 4 recognises all four Thallarcha species mentioned above, whereas Moths of Victoria nor LBH includes T. lechrioleuca. Moths of Victoria's Thallarcha sp.(1) is similar, but T. lechrioleuca seems to lack the grey forewing costal patch of that species,
There are a number of named and unnamed Thallarcha species that superficially look like this specimen that are mostly from more western parts of the continent than the ACT. ANIC has many unallocated specimens. I think we should leave the identification at genus level.