North Island New Zealand was nice, but Australia (Sydney/Melbourne) had more options of things to do. I would recommend a Sydney to Brisbane trip, spend a few days in Sydney without a rental car, then get one and drive to this hostel.
http://www.yha.com.au/hostels/details.cfm?hostelid=126
It is a little beach town that is perfect and not very far from Sydney. But that is as far north as I got since I didn't rent a car and just took the bus and ferry there.
I haven't been there yet, but Queenstown, New Zealand on the South Island would be another cool place to go.
There's a reason the beaches are empty in Darwin, Australia. They have crocodiles and jellyfish.