Melbourne being missed I can understand. There might have been masses of warheads, but not such an abundance of delivery systems to allow every such city to get hit. But Cairns, Alice Springs, Townsville and even Perth don’t make so much sense. Cairns is the worst of those.
And yeah the big aid convoy is a bit “oh we must save mother Britain”. It’s crap. Every ounce of aid that could be scrapped together would go to Sydney, anywhere else hit in Australia and of course New Zealand. India too would have very little to spare.
Perth, as Fleet Base West, kinda makes sense if you squint at it (if the writer was using a 2015 perspective without fully accounting for the difference thirty years makes, we can give a lot more leeway). Alice Springgs, as a kind of "well, Pine Gap is next door" makes more tenuous but not impossibly distant sense. Cairns and Townsville, though, are far too small. The attacks there were from an SSBN in the Arafura Sea, IIRC, so it could be hand waved as a case of "well, that's what was in range". But in that case, why skip Brisbane? Seems artificially engineered to get tension between Hawke and Joh.
The aid convoy made a little sense in the original as a "desperate Brits take half of Perth's crew hostage because the Colonies must have food squirrelled away somewhere, right?" - I took an angle that the Antipodean countries sent one convoy and never trusted Britain again after that (privately, at least; the public would probably miss the details). Certainly NZ and Australia have far more to gain from trade with one another than sending scarce tonnage on a pleasure cruise to Britain).
Mind you, republicanism probably gets quite the shot in the arm, ITTL; even after the recovery period Britain will be much further away than it was before.