We bought a trainpass and took the train from Auckland to Wellington, ferry to Picton, train to Chirstchurch and the Tranzalpine to Greymouth and back to Christchurch. It's a good idea, but it lasts only 7 days, so we had to buy a new ticket when we wanted to go back to Auckland. It is possible to take the bus - there is another pass that goes mainly by coach (bus), but that was not an option for Karen who gets carsick and Erik who uses diuretica and needs to pee more often than the busdriver.
A good option for those who can take bus is "nakedbus" check that out if you ever get to New Zealand. Very cheap. We used it to Rotorua and back.
O.K. - the train was fine. Such lovely scenery, and very confortable. But - do you know that the train between Auckland and Wellington only goes friday, Saturday and Sunday? Very strange, don't you think? And it takes 12 hours. Apart from that, a lovely ride. We ordered suites in Backpackers, and it's a good price and your get to sleep alone and have your own bathroom. Price: 70 - 80 NZ$.
We will write more specifically about Christchurch and Wellington in another entry.