Hi Alison, we researched Sepsidae on the CSIRO Diptera site, iNaturalist and ALA. There are only 16 species in 7 genera in Australia (see http://anic.ento.csiro.au/insectfamilies/biota_details.aspx?OrderID=26547&BiotaID=46374&PageID=families and ALA), and the most common one in the cooler regions of Australia is Parapalaeosepsis plebeia. There are numerous images of this species on both ALA and iNaturalist so we figure it's probably this one.
Thank you Roger for taking such an interest in Umbagong's species and bringing the wonders alive! Rosemary, Umbagong Landcare Group volunteer (among a dozen others!)