It's not just Australia that uses "foreign" aircraft for military and some other government flying. NZ, Canada, UK, USA, France, Germany, Italy, Denmark, Spain etc have all availed themselves in chartering Antonovs for example. In the Australian context who is going to set up an AOC etc etc, buy or lease one widebody long range machine for a short term low untilisation contract? I think that renting the one offs or extra capacity makes sense. Even the RAAF, USN, RAF and others have "rented" civil air tankers...
Right now we rent big firebombers [C-130 and RJ85 plus S-64/61's] as the capital cost of having them sit idle in Oz for eight months of the year far outweigh the four month "rental" from established operators. As a matter of interest Oz Air Tractors and some helicopters head to Europe in our off-fire season....that's good for Oz companies..