In World War Two, the fear of invasion reached its perceived height when the Japanese captured Singapore and started bombing Darwin. Australians lived under a fear of invasion, however the Japanese army and prime minister opposed it any plan to invade Australia, and in retrospect the Japanese army didn't have enough supply's, as the prime minister said in his last interview before execution.

"We never had enough troops to (invade Australia). We had already far out-stretched our lines of communication. We did not have the armed strength or the supply facilities to mount such a terrific extension of our already over-strained and too thinly spread forces. We expected to occupy all New Guinea, to maintain Rabaul as a holding base, and to raid Northern Australia by air. But actual physical invasion—no, at no time."

As stated they intended to cut off Australia from the US and forcing a surrender. It was thought that the Australian government had a plan in place called the Brisbane line which involved surrendering the top portion of Australian to focus on defending the main industrial areas such as Melbourne and Brisbane, advocating a scorched earth policy and guerrilla warfare. There was a massive controversy when a labor politician under John Curtain alleged that the previous government had such a plan. These allegations where fuelled by General Douglas MacArthur mentioning it in a news conference and later a royal commission into it. This strategy was seen as defeatist and treacherous by the Australian public and You can read more here. . There where no other significant strategies for a defensive of mainland Australian, although the coast was guarded by 62 guns spread around the country and the Prime minister John Curtain called back 2 AIF units intended for Burma, much to the disgust of Winton Churchill who though this was ungrateful and didn't see the invasion as a threat. This was seen as a fall back plan to protect the mainland if Kakoda fell, although due to the bravery of the militia and reinforcements that repeled the invaders in a long and bloody campaign.