Oh? Outstanding!
KC130s it is, then.
It wouldn't be super efficient, but adding that to a buy would be minimal extra costs for maximum gains.
Agreed that a dedicated tanker would be better.
For the KC-130, is there a fuel bladder in the cargo bay as well as whatever fuel the Herc usually carries? Because if not, I'd actually recommend that Oz buy nothing but KC-130s. I believe that's what the USMC currently has. Entire fleet has the refueling gear.
If you don't need it for a tanker mission, any bird can fly cargo. And if you need a tanker, any bird can fly fuel. No reason to buy an extra 4 birds just to be able to have 2 tankers available. You buy maybe 1-2 extra birds if there's some oddball mission that would require full cargo hauling plus tankage.