An alternative was proposed to cut Australia off from America, American protection and supplies.
These troop transports were to implement a plan eventually to occupy New Caledonia, Samoa and Fiji. Coral Sea put paid to that plan. A bonus was saving the Australian toe hold on New Guinea at Port Moresby. That would later be the base from which MacArthur and the Aussies would retake the island.
A very important battle.
Regarding the planned Japanese invasion of Australia:
Declassified documents reveal that the Japs intended to starve every single person on that continent to death by giving them rice that had been denuded of its nutrients.
That was their plan.
They wrote it down.