A number of posters have commented how an effort to retake Darwin helps Mac's "southern route". I beg to differ. Until the Japanese are ejected from Darwin there won't be anything going on in New Guinea other than making sure Port Moresby doesn't fall. Resources that might have gone in to the New Guinea/Solomons campaigns will be diverted to some extent to the Darwin effort. The Darwin effort is certainly going to need far less naval commitment than leapfrogging along the New Guinea coast, taking the Solomons etc. Additionally the Darwin effort is going to take time, and during this time the Central Pacific effort will be proceeding more or less on schedule as whatever extra resources might go to "Darwin" over what went to Mac anyways are things that will have little effect on the Navy approach. Armor, trucks, rails and railway equipment and much else is simply not used going from atoll to atoll.
IMHO this mean when Darwin is retaken and Mac begins his southern approach in earnest the Central Pacific drive will be far ahead in terms of closing the ring on Japan. If the Australians really want to clear the Japanese out of Darwin and other northern bits there is no way Mac will get resources for a separate drive he runs except for whatever the Australians don't want/need. The USA is NOT going to piss off the Australians, and possibly the British, by shorting their drive to liberate Darwin to favor Mac's requests.