Number plates must be registered in the same state as the car they are going to be displayed on. For example, it's not possible to attach Queensland number plates on a Victorian registered vehicle.
However, it is possible for you to purchase and own number plates from other states (to resell), you just can't display them on your car.
Make sure to filter search results by state to avoid disappointment.
New Zealand & United Kingdom
Number plates are issue country wide, so there are no state-based restrictions, however the plates must be registered in your country in order to display them on your vehicle.