Billaea is not recorded in Australia (http://www.nadsdiptera.org/Tach/WorldTachs/Genera/Gentach_ver10.pdf) - check AFD also Appears to have a well developed subscutellum, and is a reasonably largish specimen, so its likely a tachinid, but the green abdomen, grey mesonotum (with slightly green scutellum), and orange antenna is also seen in some calliphorid taxa with slightly-developed subscutella. Closest I can guess is maybe on of the Chlorotachina sp., but I'm going to leave this for time being at Tachinidae.