You plan to go to New Zealand? But you don't have a driving license or will not hire a car?
Then, you have two choices: do hitch-hike or buy to go on a bus.
Do hitch-hike could be great because you meet people and sometimes it is very interesting but sometimes, you can also be stuck in the middle of nowhere.
That is why some people prefer take the bus.
There is different compa nies which offer different type of products.
The cheapest one is Nakedbus. The fares begins from NZ$1!!! And it is true! You just have to book in advance. But be careful, someone told me not to book it at the west-coast of South Island because there work with Atomic Shuttles and if the Atomic Shuttle is full, you have to wait the next day! About Atomic Shuttles, there are very small coach (max 25 persons) and it is not comfortable so I do not recommend it.
You can also take the Intercity which is not so expensive. The buses are comfortable and the drivers are good guides. They explain you the places and sometimes stop at beautiful places where you can take some pictures.
There is also the Kiwi Experience where there is a lots of young backpackers. The buses are comfortable and also new ones.
And another one which is the Magic Bus. I did not get into one but by the outside, it looked very old and not comfortable.
All I can say is that Intercity and Nakedbus could be more interesting even if there are passes. So, before booking any pass, calculate how much it will cost you without these.