I don't know about the rest of the guys around the world, but most have previously worked with TAAATS, so the bridging course required to catchup, would be extremely short, surely no more than a couple of weeks. In contrast to this requirement, you are employing ATC'ers who have been exposed to traffic levels far in excess of anything you would see in Australia, and exposure to situations (eg. constant U.S aircraft carrier ops, and fighter/refueller ops.) that guys would never get in Oz. This must be a huge asset for ASA into the future.
Instead of this option, they go the ab-initio route, and spend a year and a half (and around 300 to 400 thosand dollars) training someone, who even if they check out will take years (if ever) to gain similar knowledge and experience to the expats.
Seems like a no-brainer.