If your number plates are stolen, you'll need to apply for new plates, which will involve paying a fee. There's also the added hassle of changing your numberplate details on your registration, insurance and any interstate toll accounts.
About the anti-theft screws: The anti-theft screws come in four sizes, to suit all types of vehicles. They are fitted with a screwdriver but require a special removal tool to remove them (your local mechanic and some police stations will have the tools to remove them if required).
Image Source: Wanneroo Police on Twitter (https://twitter.com/WannerooPolice/status/917196179326115840)
Highly recommend doing this. We had numberplates stolen from our car twice in two months last year. It was very inconvenient, and had we known about the special screws earlier, we would have saved a lot of trouble and expense.