Do you really think the C-17s are there just for the RAAF? They are a national asset that just happens to wear RAAF roundels...
they're considered to be joint assets. in fact there are a few wags that consider them to be the part time property of DFAT and DIm(m)i
RAAF have generally been better at getting assets as they have read the tea leaves prior to Gate Reviews and argued dual use. The same can be said of Bill and Ben Mk2, and Choules.
Unfort army don't have that get out of gaol card to use in front of Central Agencies etc..... (although Bill and Ben Mk2 did get a good run and plug by Army stemming from the ET lessons)