Number plates are an identificatory item on the front and rear of a car that are unique to it, providing an identification reference point. When photographing cars, the question is always whether to remove them or leave them. In this article I aim to provide light on whether or not you should.
Type of Photography
The nature of the photography that you do plays a big part in this.
Events - When you take photos of cars at events the cars have been especially put there to be displayed so photography including number plates is okay.
Private Shoots - When you do a private shoot, the owner sanctions you to take photographs of their vehicle so once again photography including number plates is okay.
Spotting - Spotting is slightly hazy. If legislation is correctly followed then it should be okay unless a car's owner objects.
It all boils down to a question of consent. If the owner consents to the photography then it is okay, but if someone were to specifically ask for removal of either the number plate or all photos of that particular car you need to do so.
With regard to people, Australian legislation states that you must obtain consent from someone if you intend to use the photo for commecial purposes, to make money. You are permitted to take photos of them regardless if they are on private property or not, as long as you are on public property, or have permission of the land owner of said private property.
By applying this same theory to license plates we can see a set of guidelines, but when in doubt, just ask. Most people are happy for you to do so.