what is to stop someone like Adagold using ( as an extreme example ) Garuda ?
The brokers must ONLY supply aircraft and operators licenced and registered by JAA, FAA, NZ CAA or CASA. Anyone else they want to use would have to be approved specifically by Defence.
Is anyone actually asking why in gods name an Australian Defence Force with the interests of its Australian troops and its own security let a tender out to third party and brokers?
Defence have something called the Standing Air Panel. This is formed from a tender of interested parties who can provide Defence with charter aircraft as required - (the requirement could range from a Caravan to an AN225). Some of those who tendered to be on this panel were aircraft operators and some were brokers. About 8 were selected to be on the panel (can't remember the exact amount, but it is on the Austender website - it is not a secret).
When Defence need civilian airlift they go to the panel and ask for offers and then select what suits them best. For the MEAO sustainment, Strategic would have offered their A330 (although in previous years they offered the HiFly A330). Adagold apparently offered a HiFly A340 and other panel members no doubt have offered other solutions.
Interestingly, neither Qantas, Virgin, ANZ or Cobham are on the panel, so none of them are allowed to tender for any Defence work directly - it goes through panel members. However that doesn't mean that a broker couldn't offer their aircraft to Defence, but they can't do it directly. I have no idea if they didn't bother to bid to be on the panel or they weren't selected.