If he's getting evacuated by Submarine, I personally wonder if he will still end up in Australia instead of Hawaii; was Freemantle or other bases even available in March?
I'm not sure what you mean by "available" but given that the USN moved there on March 1st I'd wager that the infrastructure was well in place by mid-February
Theoretically there doesn't even need to be a dock open, Mac and company could just take a dingy and row, sorry *be rowed* to shore.
On a separate note. From what I remember of the Japanese invasion plans, they planned to take and hold Darwin but weren't going to go that far south. They were certainly going to stop well short of Katherine NT. So keep in mind that there is a railway from Larrimah, NT to Katherine already in place. There are also airfields at Daly Waters NT and Katherine NT which were used in OTL and can be/were expanded among others.
I also wonder if the Stuart Highway will be allowed to deteriorate as much as it did in OTL. Unlike OTL in TTL, it's going to be the primary logistical lifeline to a (politically) vital military front. If the Japanese invade in February, the Allies months of good weather to use the road and begin work on improving its durability before the monsoon season starts. Sure construction will have to resume in the fall but I highly doubt that they're going to be starting from scratch. The same thing goes for railway construction.
I also highly doubt that the Japanese are going to be pouring large numbers of troops into Darwin in 1942 due to strategic concerns elsewhere and the poor port facilities on Darwin. The more I think about it, the more I think that the Japanese position in Darwin is going to be extremely weak. Their occupation of the Top End is going to be focused on Darwin proper and the Stuart Highway/Railway. Given that the IJA is going to kick and scream against any allocations of troops/assets into the region, I highly doubt that many if any motor vehicles are going to be sent in TTL. The garrison force is going to have to make use of whatever they can capture and given the notoriously poor quality of Japanese support staff during WWII, I highly doubt that these captured vehicles/locomotives are going to last that long. A lack of motor vehicle transport is going to severely limit Japanese force projection outside of the immediate environs of Darwin itself. I doubt it's going to take much to push the Japanese to the coast, retaking Darwin is going to be an entirely different prospect.
There's also the very real possibility that the Japanese decide to cut their losses and evacuate/abandon Darwin at some point instead of pouring resources in for a protracted fight.