I hope I don't rape the thread intent too badly with this, but ...
Remember discussing this a couple years ago in context of the Japanese perspective from Senshi Sosho. Invading Australia would be a nightmare. Weren't enough troops (would have required 12 Divisions, minimum) but most important, there wasn't enough shipping. Planning schedules said 1,500,000 tons (at least) to keep things going for a descent on OZ. Since the Empire only had 3,000,000 tons, that would have been painful.
The IJA affirmatively recognized it would be a slippery slope, "Reflecting on the bitter experience of the China Incident, the chances are high that an invasion would eventually extend over the whole of the Australian continent." (Ugaki). According to MajGen Tanaka Shin'ichi, "Even if the Army could supply the required strength for the proposal to invade Australia, there are grave fears that the problem of supplying shipping would destroy the fundamental basis for executing the war." Tanaka, Tanaka Shin'ichi Shosho no nisshi no motozuku shuki. Some of the guys in the IJA Division of IJGHQ weren't all that stupid. They implicitly recognized that the entire Eastern coast of the sub-continent would have to be conquered and that was simply impossible.
So if you don't want your entire body ground into hamburger, don't stick your winkie into the wood-chipper.
Darwin wasn't the best place, either to invade from, or invade to; just too isolated. But it did confront Timor/Ambon, so if was a backwater, it was an important one, nevertheless.