Taking a car on the ferry?
Not a lot of point in doing that as you would normally drop the car in wellington(north island),jump on the ferry and then pick up a new rental as you get off the ferry in Picton(south island) It's about about a 100metre walk.
Also a lot cheaper than putting a car on the ferry$$$


On a couple of occasions I have moved cars from Wellington to Auckland for Thrifty.
Don't remember paying anything and they give you a couple of days to do it.

Cheers