Unlike most of the Pemphredoninae, Psenulus has 3 submarginal cells. Note too the distinctly petiolated metasoma.

Image courtesy of: Graeme Cocks
https://inaturalist.ala.org.au/observations/28895056

Note the distinctly petiolate gaster. The distinctive colour pattern makes this northern (e.g. QLD) species readily recognisable.

Image courtesy of: Kai Squires
https://inaturalist.ala.org.au/observations/203404918