These beetles have been identified by Ken Walker as Cadmus (Brachycaulus) spp. I wonder if they are a pair, because the antennae on the smaller beetle is so much longer and I would assume therefore that it would be a male and the larger beetle a female.
These can vary from copper-gold to a deep blue colour. This one a green-blue female. About 25mm long overall and just clinging to the topmost part of a small spiny acacia. In a local nature reserve. Glenfern Valley Reserve.