Following Bergmann's rule, colder climate animals tend to be larger, in terms of Mass northerly tropical birds can be 1/3 smaller at 200grams versus 300grams in the south. This individual from the gold coast hinterland is probably somewhere in between as it was roosting in a subtropical environment. I would think the same correlation would be present across the wattlebird genus as the tasmania wattlebirds are much larger than the little wattlebirds of the north Brisbane region.