Have been following this thread, and biting my tongue until now, so here's my take.
1./ I'm not a Territorian, I'm a Victorian, which is the other end of the continent However, the Territory, and it's West Australian equivalent the Kimberly, have been a life long interest, and I've visited these areas several times.
2./ The biggest defence is distance, and a lethal environment. i.e. Desert, and native killers.
You don't swim in the sea because of crocs, sharks, jellyfish, and other lethal marine organisms.
You swim in inland rivers, and rock pools, giving due regard to crocs, and snakes.
3./ From personal observation, I've seen big saltwater crocs as far inland as Timber Creek, and Katherine, which are both well up the respective river systems.
4./ The North Australia Observation Unit - aka the " Nackeroos," or "Curtin's Cowboys" (after John Curtin, the then Prime Minister,) a unit of the Australan Light Horse, mostly made up of locally recruit stockmen, supplemented by experienced bushmen from other parts of the country, assisted by the local Aborigines.
5./ The almost "schizophrenic" decision by the Northern Territory Police, soon after the bombing started, after spending decades, tracking down, and locking up any Aborigine who had speared or attacked a Japanese fisherman / pearler, threw open the gates of the old Fannie Bay Goal, and told them to go home, get their spears, and start "hunting Japs."
Some of the above is somewhat appochryphal, especially the last point.
But my point is that any Japanese force trying to move away from very close to Darwin faces a "world of hurt", even before they engage regular troops.
All of the 200 odd posts,in this thread, about roads, and railways, are all doable, given time, manpower, equipment, and weather.
However, while waiting for the AIF to get back from the Middle East, the Light Horse have a long history of operating in that type of environment, as evidenced in the Boer War in South Africa, and particularly in Palestine. If you get a chance, or have the interest look up two films "The Lighthorsemen", and "Forty Thousand Horsemen" (the latter has the distinction of being made by the nephew of the Commander of the Light Horse, at the battle of Bersheba, in 1917.)
They have the capacity to contain the Japanese into a defined area until everything else is ready to push them out.
If you want another example of of the ability of barely trained Australian Militia to contain the Japanese, until the "Regulars" arrived, have a look at the action of the 39th Battalion, on the Kokoda Track, in New Guinea.
Apology for the rant, but most of you appear to have missed the point, or underestimated the environment.