AFAIK, you only need a permit if the car is unregistered and you brought it over. Otherwise your free to drive around on the NSW rego until it's due. I was told something about only being allowed to drive it here for 3 months, but it's not really something they can check on, it's more a recommendation because it would mean the car is probably going to be a permanent WA car, so it should be WA rego'd.