i have used ACE rentals in christchurch for 7 years. very happy with the cars and service. they pick you up at the airport door in their shuttle bus and take you to their depot which is only 2 minutes drive away.
US fuel prices at $2.90 USD Gal
NZ fuel at $1.65NZD/lt
Exchange rate at 1.48 USD to NZ
You will be paying 1.14 USD/lt or $4.21 USD/Gal
So about a 40% hike....remember though unless you hire the biggest car/van or ute they have you will use far less fuel than the average US vehicle. So expect parity or less on a per mile basis. And South NZ is a bloody tiny little island!
Last time I went I booked well in advance through Economy Car Rentals (meta agent) and got a i20 from Lucky Rentals (Christchurch Airport) for 8 days costing $160 NZD.
If you fly into Christchurch all the car rental establishment are not in the airport. You go out of the terminal and wait near the bus stand then a mini bus service picks you up and runs you the mile or so to the rental yard.
We went there for Christmas and rentals were expensive. I guess its the high season there so they can go down in price at other times of the year. I rented from Avis but there are lots of options it seemed. We looked at the camper vans but it didnt seem to save us much money vs a hotel + car and honestly, I didnt see all that many places I would have actually wanted to park and stay.
Since they drive on the LEFT side of the road, I made sure I got an automatic since I didnt think I would be able to reverse learn how to drive a manual in the mountains and while on the opposite side of the road.
Gas was well over $2.00/litre. Most Chase cards dont charge foreign transaction fees which was nice. We drove all over the South Island for 10 days. Feel free to hit me up with any specific questions.