If, during your trip to New Zealand you want to cross South Island from east to west (or vice versa), then you'll probably end up on Arthur's Pass, a hikers' paradise where keas rule!
Some good news - you can easily get to Arthur's Pass, no matter what form of transport you're using. Whereas you need a vehicle to reach some areas of New Zealand, here you can come by train, car or even by hitchhiking, which works just as well.
If you like traveling slowly across stunning landscapes then take the train from Christchurch, to the east or Greymouth, to the west. The views are equally impressive from either end. The approach to Arthur's Park Station is breathtaking ; a staggering alpine welcome! Bear in mind that ticket prices are more or less the same, no matter what distance you travel, so price-wise it's worth going down the whole line - you won't be wasting time nor money.
That said, the journey by car is just as enjoyable. The views from the road are stunning, as it winds its way up and down the valleys. If the weather's good then I suggest you stop along the way; the mountain views are particularly impressive! But keep your eyes out for thieves. They come in the form of kea birds and they love anything shiny...don't turn your back for too long!
Once you arrive at Arthur's Pass you've got two options. You can explore the region or head straight off to see other places in New Zealand. If you're here for a look-see only and are not a hiking fan, then don't waste your time. However, if mountain treks are your thing, then you'll find a range of accommodation from hostels to hotels to campsites. Check out the trail in advance, keep an eye on the weather and sleep well; the summits await!
Some of the Arthur's Pass treks are not suitable for all. Choose the right ones according to your physical condition (be realistic!), your equipment and your level of experience. If you're not sure about which options to take, you can always drop into the local guides' office. They'll be able to advise you and suggest hikes that meet your needs.
If you have the budget, you can always hire one. They are as competent as they are passionate about the area and have insider knowledge of the all that the local environment has to offer. Whatever the case, go in summer rather than winter, when heavy snow makes it difficult and dangerous for hikers.