re: Were the Japanese planning to invade Australia during WW2?

No, their problem lied in the fact that they relied on other places for resources. That's one reason why they expanded out so much. They had a problem in China but weren't necessarily bogged down, they just didn't have the manufacturing capabilities/resources to be involved in a long, drawn-out war. Where do you think they got the steel/oil/etc. before WWII started to build those carriers? The U.S. and other countries...once that supply stopped, every loss thereafter was devastating because they could not replace it. Australia would definitely have been invaded/taken due to the fact of the staging area for allied forces.