Avoid the Kiwi Experience to any price! You meet so much fun people in the hostels anyway, so having the opportunity to relax and sleep in the bus is a bonus. Therefore, try out Intercity Coaches:
They go everywhere and provide the services you need, but is a lot cheaper than the Kiwi Experience.
Even cheaper and a lot more fun is hitchhiking. That's when you really meet the fun people and not just only drunk 16 year olds...
Take care and good luck!