While I dont know this for sure, an old workmate told me he had an issue with P plates which he put to the police to find out what the case was.
Legally its stated as P plates have to be clearly viewable on the outside of the vechile. So with tinted windows, you really arent supposed to have them on the inside of the car at all. Especially how some pople hide them in corners and so forth. So technically if you have tinted windows or read visors, etc you should display it outside the vechile.
PS: Go to Sydney, they are all outside the cars