The process was quite convenient. You just need to drop the key in the small house at the exit when returning the car. The road assistance is just a card with a contact number, costing 10 NZD/day, which I found useless and a waste of money. Because traveling to Milford Sound requires you to rent snow chains, you have no choice but to rent them for 12 NZD/day. If the store doesn't explicitly say you need to rent them, don't bother.