The invasion of Papua New Guinea was seen as a very real threat to Australia's security, as defence of PNG and Port Moresby was critical to victory in the south Pacific. Had Port Moresby fallen, it would have left northern Australia more vulnerable to attack. Singapore had already fallen, Rabaul (PNG) had already fallen, and the Japanese troops were getting much closer. Over the period of a year or more, Darwin and northern parts of Australia experienced periodic bombings from the Japanese.
In May 1942, a Japanese invasion fleet departed Rabaul for Port Moresby, and the Battle of the Coral Sea began. It was a very real threat which was only turned back by the US aircraft leaving from carriers. After being turned back by the US, the Japanese then turned their attention to an attack over the Owen Stanley Range via the Kokoda Track, which linked the northern and southern coasts of Papua New Guinea. Thanks to the Papua New Guinean natives assisting the Australians and the US troops, the Japanese were turned back, having to retreat to bases at Buna, Gona and Sanananda, where they were eventually defeated.
What is north of Australia with dense forest
Is there sailing from Papua New Guinea to Australia
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.
New Guinea is 150 km from mainland Australia. New Zealand is 1250 km from mainland Australia.
Australia - Papua New Guinea flight information. Link.
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,
Torres Strait lies between Australia and Papua New Guinea.
Australia?
The three seas which border both New Guinea and Australia are the Arafura Sea, Coral Sea and Timor Sea.The body of water between the northern tip of Australia and the southern coast of Papua New Guinea is Torres Strait.
The Coral Sea borders both Papua New Guinea and Australia.
Papua New Guinea was Australian territory.
The Solomon Islands are northeast of Australia and east of Papua New Guinea.