Actually, from what I’ve seen, the Australians are stopping that pretty well. Am I mistaken?
Moslems are allowed to come in.
Yes, the current government is.
The system isn’t perfect, but any person who is assessed as a potential threat is not given a visa. Anybody who tries to get in without a visa is placed in detention - often off shore.
But the system wasn’t always working as well as it does now, and even now it’s not foolproof.