Actually this is not "illegal" in Australia at all. Australia is made up of states with different legislation sames as the USA. In some of the states a person may "use reasonable force, that they personally consider reasonable, to defend themselves, other people and property against threatening intruders".

So in South Aust for example, how does this work?

If a burglar enters your house and you think you are in danger you can defend yourself with a firearm.

Obviously if you shoot a fleeing burglar in the back you will have legal problems.

You can not own a firearm for this purpose though, it must have a sporting or business reason for ownership.

(PS I am not giving legal advice above.)