Plan to rent a car or camper in Aukland and drive to the southern island. I read somewhere that you

Plan to rent a car or camper in Aukland and drive to the southern island. I read somewhere that you don't take rental cars on the ferry. Is this ture? Also some car rental agencies seem to have a surcharge for a drop off on the souther island. Any ideas to make this affordable? Any recommendations on agencies? thank you.
It depends on the car rental company. Some won't allow you to take their cars on the ferry others will. The ones that won't will arrange a replacement car on the other end, just arrange all this when you book. I believe APEX will let you take their cars on the ferry and will even give you the ferry ticket for rentals longer than 10 days.
My long reply: We rented from Apex last year in Auckland -- a very nice Ford Focus in excellent condition, with a reasonable priced GPS we named Ethel. We drove north first to Cape Reinga,then worked our way south and crossed by ferry from Wellington to Picton. AFAIR, the rental price included car crossing on the ferry so we just had to buy 2 passenger tickets. They took care of all those arrangements, and everything was seamless. We liked that idea, which meant we didn't have to empty and return the car in Wellington then schlep all our stuff on the ferry and reload in a new car.
Drove around South Island, and finished up in Queenstown with a quick and easy drop-off after 3500 kms. As it happened, when I was making the initial reservation I mixed up the dropoff date and said we would be a day earlier. Got a polite email that evening wondering where the car was, so I called immediately and left a message to explain. No problem barcelona the mediterranean cruise when we dropped barcelona the mediterranean cruise the car off at Queenstown airport, just a normal charge for the extra day. Even the pickup in Auckland was easy: they came to pick us up at the hotel and drove us to the office to complete paperwork. You can probably tell that we were very pleased with our Apex experience, and the price was very competitive.