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Concierge.com's insider take:
The South Island has six multi-day Great Walks, all maintained by the Department of Conservation. They range from the remote and challenging 22-mile Rakiura Track on Stewart Island (off the southern coast) to the easygoing, beachy, but more heavily trafficked 31-mile Abel Tasman Coastal Track. The Milford, Kepler, and Routeburn Tracks are the most popular; all wind through the exceptional scenery of Fiordland. In the northwest, the 51-mile Heaphy Track, running from Golden Bay near Nelson across to the west coast near Karamea, is a mix of high plateau, forest, and shore walking.
All these tracks take a minimum of three days, require a good level of fitness, and are best for active types who are used to lugging all their own gear, camping out in basic huts, and getting wet when it rains. You'll need to buy a Great Walks Pass before your trek in order to use these huts and campsites; you can get one (along with maps, directions, and trekking safety tips) online.