They can’t, they’re Australian citizens. This is the problem with most of the EU and Australia, we have visa wavers with them and any of their citizens can jump on an aircraft with only a valid passport without any consulate screening. We can only hope these people are on some kind of watch or no fly list.
“They can’t, they’re Australian citizens.”
Not can’t, won’t. Nations have been revoking citizenships since it was invented and they can easily do so here. The problem is out-of-date laws and a lack of political will to change them (i.e. political correctness).