Though the Milford Track gets more recognition, nearby Routeburn Track might be even better.
Kyle Ellison – trip report:
… This past year Lonely Planet listed the Routeburn Track as one of it’s top ten treks in the world, and the heavily trodden track has seen it’s annual numbers climb to over 13,000 walkers per year.
… the Department of Conservation limits the number of people who can through-hike the 20-mile route by only providing 50 beds in each of the 4 backcountry huts scattered along the trail. During the summer months, the no-frills huts (mattresses and gas stoves are provided) run a pricey $40 US per person/night and reservations are absolutely crucial. …
Permits are needed. Click over to Routeburn Track — official website — for information on how to tramp independently or guided.
There are plenty of good trekking guidebooks. Amy McGinnis recommends one I’ve not used — NZ Frenzy: New Zealand South Island by Scott Cook.