That, for me, is a classic example of the hand ball/no hand ball conundrum:
Yes, the ball clearly hit his arm BUT it also (for me) clearly wasn’t intentional (as he didn’t have time to move his arm out of the way after the ball came off the Aussie head), BUT he did have his arm well out away from his body, BUT he was contesting/jumping for a head ball and the arms kind of naturally come up for that action BUT he was in the penalty area and it was at least a possible goal scoring situation that his arm clearly interfered with....
Final judgment (for me) - Penalty kick was correct to give. No yellow should have been given though.
Well thought out.