Gidday Milo, [ or should that be Jetsetter ] you can take your hire car from one island to the next, BUT you have to put it on the Ferry to crossover, we are also planning to do that trip next year, it looks fantastic ! see ya dazren
Yes milo you can. You would fly into Auckland, hire the camper. Spend a couple of weeks driving around the north island. Then when you get to Wellington at the bottom of the north island you will catch the ferry across to Picton at the top of the south island. It takes about 4 hours I think from memory. A long time ago since I did the trip. I think it was 1972. Then you would see as much as you want to of the south island and hand your camper back in at Chch airport and fly home from there. There is a lot to see on both islands. I estimate you would need at least 2 weeks for each island. Any less would not do it justice. You can get package deals to do all that in one price.
Check with your friendly travel agent. I often see Fly/Drive deals advertised. Some are motorhome/campervan deals, others car rental and stay in motel/hotel accommodation. Be careful with hotel/motel deals as you could possibly get "locked into" a deal with a specific hotel/motel chain.
as HW said, some of the hire companies throw in the cost of the ferry just to get your buisness, shop around for your deal, NZ is a great place to visit.
We did the fly/drive about 4 years ago. hired a car not a motorhome as when I did my homework found it was cheaper with a car & used a cabin in caravan parks. We did both islands no rouble. HAd a grat time.