Bookable parking has now ended for the 2020 winter season, so you will be able to park on mountain overnight without a booking.
If you are arriving during the day when the Ski Area is open, you will be permitted to drive up the road if the car park has available capacity. If it is full, you may need to wait for someone to leave before you can drive up.
Overnight parking at Whakapapa is detailed in red text on the map below: