Although the Japanese never planned to invade Australia, they did plan to isolate it from the rest of the world and perhaps, bully it into giving up the war. As part of their strategy, they landed troops on the then-Australian territory of Papua and attempted to capture Port Moresby by going through the Kokoda Trail (or Track, if you're really that picky). However, this offensive was halted by an Australian-American force.
But what if the Japanese were able to capture Port Moresby?