To defend its territory in Papua New Guinea against a Japanese attack.
All other positions of defence between Australia and Japan had already fallen. This included Singapore and the Malay Peninsula. Essentially, Papua New Guinea was seen as the last bastion of defence between Australia and a Japanese invasion. If Japan took control of Port Moresby, in the south, it was regarded that it would be very easy for them to launch both air and sea attacks on Australia.
The word "guinea pig" in Japanese is "モルモット" (morumotto).
The were ON Australian territory since Papua New Guinea was administered by Australia.
Is there sailing from Papua New Guinea to Australia
Papua New Guinea was important to Japan because they needed a piece of land close enough to Australia so their aircraft would be able to bomb cities and major targets. So Port Morseby was a great place to stage the invasion fleet.Answer Japan had never seriously considered invading Australia, and desired PNG to deny it to the Allies and to neutralise Australia as a base for counter-attack. PNG was important to Australia as a source of raw materials, and was under Australian control under a League of Nations mandate. The Japanese presence there was a huge worry to Australians, who weren't aware that there were no serious Japanese plans to invade Australia.
Yes they did. The invading forces at New Guinea namely Kokoda, were to be used in the invasion of Australia. The Japanese planned to take the northern cities first like Cairns, Darwin, and Townsville and then they planned to take the southern cities of Brisbane, Sydney, and Melbourne.
Australia?
Torres Strait lies between Australia and Papua New Guinea.
In the east, Australia and Papua New Guinea are separated by Torres Strait. The Western half of the island of New Guinea is separated from Australia by the Arafura Sea,
Australia - Papua New Guinea flight information. Link.
I'm not sure as to whether the Battle of the Bismarck Sea was the pivotal point but the Japanese had such tremendous losses there that they could not reinforce their troops at New Guinea . See the related link below .