i'd suggest contacting them first - would be kind of frustrating to fly up here only to find out they wont release the car (or that customs is shut for the weekends)
They're at:
McArthur Av, Hamilton 4007
(07) 3868 9591
open the URL, zoom out about 4 levels and you'll see that the airport is about 2 suburbs away to the north-east- cab would be 10-15 depending on traffic.