They'd have to ban non-travellers from going through security.
Most likely solution is to check on boarding. From memory that's how they do it on intra-Europe flights. Will slow the process down though, hence why QF won't be happy.
When i was flying back from hongkong, they would do a passport check while people were lining up before the gates opened for boarding, and stamped the boarding pass. could do something like that here.