Why was Papua New Guinea important to Australia and Japan in World War 2?

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.