Hi,
I did a trip to the South Island Last October and used Ace Rentals
which I was really happy with.
They are a big company, have been around for a while and their prices
are very good. Like most car rental companies they have a depot just
outside the airport and will pick you up in a shuttle bus when you
arrive and take you to the depot to do the paperwork. Likewise they
will drop you off at the airport after returning the car.
This is to get around the high charges that the airport authorities
impose on the car rental companies, which of course gets passed on to
the customer.
Ace Rentals head office is in Auckland. You should be able to find
their website with Google.
Enjoy your trip.