Why is yr right wrong?
You can do just about anything on your own property, unlicensed underage drivers, drunk driving, modified cars un register able. All that is OK, so why not unlicensed pilots in unregistered aircraft as long as they are below 500' and over their own property as per powerin's post?
It would depend on how high in Australia your property rights go up to. In the USA they have a specific altitude that you own up to, (hence why drones over there are a issue). Not sure what it is here, but there would be some limit.