This is the first insect of this type that I've seen so am unsure what it is. It doesn't appear to have halteres so I've placed it under the wasp category for now. It was around 12mm in length and initially looked like an ant. Thanks to Marcus Leach for the initial sighting on a handrail in ANBG's Rainforest Gully.
See ALA gallery, especially the mating pair: https://bie.ala.org.au/species/urn:lsid:biodiversity.org.au:afd.taxon:0ddce48f-82de-4b00-839a-d2f98ae2daa8#gallery