Rather than trying to work out who might be a threat or not on the basis of some religion which any immigrant would just lie about, the solution is to just stop all immigration.
If Australia tried to bring in something equivalent to the white Australia police again we would rightly become international outcasts and we would end up being treated by the international community something like apartheid-era South Africa.
If we just choose to end the "big australia" policy being promoted by real estate agents and developers then there would be no condemnation and no bad people would come in either.
