It is a leaf beetle (CHRYSOMELIDAE), most likely of the subfamily Eumolpinae. It might be in the genus Edusella, but I have a bit of work to do before I’d confidently take the ID to genus, let alone species. But Edusella is a reasonable first-guess, as they are known prey of Cerceris antipodes (Evans & Hook, 1986), they do look quite similar (see iNat sightings of E. perplexa, for example), and some species feed on the leaves of Acacia. We certainly have plenty of wattles within easy reach of the Cerceris nests!

A little later, when I next checked, the burrow was open again. A prey-laden wasp approached but instantly backed off, dropping her prey near the entrance. Almost immediately another wasp reached out from the burrow and dragged the beetle below!