Do Christchurch—Auckland is a disappointment, especially after visiting Sydney or Melbourne. You can take an express bus from Christchurch to Queenstown and its about 8 hours. Its touristy and spendy but I sure had fun there. Air New Zealand might have some cheap airfare specials. The south island has the most variety and craziest weather so pack options. I would highly recommend getting to Abel Tasman National Park, via Kaikoura, if you are renting a car. Its at the top of the south island and you can see the North Island across the channel. ABSOLUTELY amazing place... lots of outdoorsy things (tramping, kayaking, beaching) but not much in the way of lodging or food so its a camper-van spot.