Australia has put in a request for price info for a 6th C-17.
The additional aircraft will probably be useful. As a United States Air Force C-17 pilot told me once, "We take a lot of business away from the C-130".
Additional C-17 buys were never mentioned in the joke that is the last Defence White Paper which some hold so dear. This points back toward the inability of Defence bureaucrats to consistently perform good long range planning.
Given the way that additional buys of C-17s now creep into budgets (just like the 5th one), it may be time for a top to bottom review of all ADF airlift needs; a clean sheet of paper.