It depends on where you will be staying and what you want to do. There is a lot to explore in New Zealand, so if your intention is to go further afield than just touring the city, a rental car (or campervan, another popular choice) a great way to go. On the other hand, if you’ll mostly be hanging out within the city, you can get to a lot of places on the city bus. In this scenario it may not be worth the hassle to have a car.