As the name implies this bug lives on the small tree called Cherry Ballart. For many decades there has been only one of these trees in the park, and there are probably no others for a considerable distance. The bug has managed to survive despite this. Judging by photos on the web the bug is occasionally seen on other plants, but I could not find any literature about it's biology or host plants.
Classification
from Australian Faunal Directory
Order Hemiptera - bugs, hoppers and scale insects
Suborder Heteroptera - true bugs, 2806 named species in Australia
Superfamily PENTATOMOIDEA - Shield Bugs, 572 named species in Australia
Family PENTATOMIDAE - Stink bugs, 364 species in Australia
Genus Commius
Species Commius elegans