I was hoping that if the Japanese had taken Darwin the Australians would have extended the Central Australian Railway from Alice Springs to Darwin.
I'd not heard of the Great Western Railway of Queensland before. What are the chances of that being completed to support the forces retaking Darwin ITTL?
Though I think completing the Central Australian Railway prior to retaking Darwin is a bit much, the experience and psychological/political trauma of having Darwin taken probably means that at least one railway gets completed after Darwin is retaken, probably after the war.
IMO the most likely POD would be having the Japanese decide to attack Darwin instead of Timor and Bali right around the time they launch their carrier attack on the harbour (Feb 19/20). This was briefly considered IIRC as it would accomplish the same goal of cutting off aerial reinforcements to Java while having a considerable strategic/political upside in threatening Australia (that being said the logistical problems mentioned above probably led to the plan's cancellation in OTL). Assuming they invade on the 19th that means that the invasion predates the departure of the
Langley and
Sea Witch. With Java totally cut off and Australia threatened by Japanese bombers out of Darwin there's no way that they get sent. Instead, in TTL, the pilots and their aircraft remain in Australia and likely wrack up some needed experience resisting Japanese bomber raids alongside the aforementioned aerial defences. The US might also send more pilots and ground crew that in OTL were diverted from the DEI to India to Australia in TTL due to political pressure (51st FG perhaps other elements of the 10th Air Force).
Taking Darwin also would have an interesting effect on MacArthur's retreat from the Philippines. The US would probably still try to evacuate him, only in TTL it would have to be via submarine instead of by aircraft. I also wonder if it might be an earlier death blow to the remnants of ABDA command with a decisive breach of the Malay Barrier. It could result in the allies deciding to cut their losses in the DEI and trying to get as many ships and troops as possible out earlier.
Timor and Bali still would need to be taken in TTL so that likely results in either fewer Japanese reinforcements going to Burma following the fall of Java or perhaps a delay in their operations in the Eastern DEI/ Iran Jaya, perhaps both. The Japanese might not push north from Rangoon in TTL, they didn't plan to in OTL but did so because the strategic situation allowed for it. In TTL they've expanded beyond their original goals in a different way and need to allocate the necessary shipping to support it properly. There's also a question of where the aircraft come from to pursue a bombing campaign in Australia and how long the Japanese keep it up. I have a hard time seeing them absorbing the casualties that would come with unescorted daytime raids for very long. Might they resort to terror bombing at night as a way to divert even more Allied resources? Or do they simply transform Darwin into another garrison and turn their focus elsewhere (my guess is the latter).
All this to say that the invasion of Darwin probably doesn't prolong the war but it could lead to the Pacific taking a very different course. If the Japanese don't push north from Burma before the onset of the monsoon season the different strategic situation totally changes that campaign and could very well shorten the war.