Note that there are various colour forms of the nymphs and the stage should be judged by the size of the wing buds. 3rd instars are about 15mm long.
Classification
from Australian Faunal Directory
Order Hemiptera - bugs, hoppers and scale insects
Suborder Heteroptera - true bugs, 2806 named species in Australia
Family COREIDAE - Crusader Bugs or Squash bugs, 92 species in Australia
Genus Amorbus
Species Amorbus robustus