Boshio good for you that you had the foresight to have a pv system installed under the NEM program.  I'm sure you must be happy with the money you are saving.  Good to hear you are also doing your part to help by investing in an electric vehicle.

Regarding the issue of adding more panels to cover the added electricity required to charge your car, you can add more panels and maintain you NEM provided you don't export any power to the grid from the new panels by installing batteries with the new panels.  You should call your pv installer and they should be able to help you.