I think it's a Queen Butterfly with all of those white dots.
Butterflies and Moths of North America | collecting and sharing data about Lepidoptera
I'm not sure they have that one in Australia. From what I can see its what the aussies call a Wanderer aka a Monarch. Apparently they might have colonised Australia in the 1870's as a result of some cyclones carrying them across the Coral Sea Wanderer or Monarch butterfly: Museum Victoria