This is virtually ASB in my opinion. While both the United States and Australia have undeniable histories of treating indigenous and (in the case of the US) African slaves in genocidal ways, these had to do with the treatment of what both nations saw at the time as inferior or primitive people within their own borders. Not justifiable by any means, but possibly understandable given prevailing western attitudes in the 19th and early 20th centuries.
Japan, for all its war crimes, was a modern independent nation that had been beaten in a war involving an alliance of many nations. It was also a war in which both the US and Australia claimed to follow international and military laws - and that was the official policy, regardless of the fact that they weren't followed always in actual combat with Japanese soldiers. Also, Japan had been flattened and millions of its citizens had already been killed and left homeless by US bombing and blockade. While the MacArthur occupation was amazingly benign and farsighted given US wartime attitudes, I suspect the typical Australians would react similarly in a similar role. They would not support a policy of deliberate brutality against the Japanese civilian population - especially considering the virtual absence of outright resistance (or even hostility) to the occupation by the average Japanese civilians.