winniewoo- hope you're still watching this thread!
We used About New Zealand car hire. They were very nice- excellent customer service. Like most (all?) the other car hire companies the cars are old, not new ones.
We had booked an Economy hatch, which was a Toyota Corolla, and expected it to have split rear seats. It didn't. Only one case fit in the boot so we had the other case and bags on the back seat and the pushchair across the footwell between the front and back seats, which was a pain.
It was horribly underpowered. We hadn't realised just how many hills there are and how many twists and turns. We looked in a car lot and most of the cars on sale seemed to have 5 litre engines
Driving in NZ was an eye opener. The roads weren't particularly busy but the maximum speed limit is 100 km/hr, and on some roads it's 80 or 70. You get nowhere fast!
With hindsight we wouldn't have driven at all but based ourselves in one place. Mum went everywhere by coach and saw more than we did. We spent more time driving than actually seeing the sights.
So I would recommend the company, but get a car bigger than you think you'll need.