Advance reservations are required and it's $30-40 per night. This is probably the biggest shock to the system for American hikers used to wilderness camping. One thing to remember is that DOC could easily charge double that amount the demand is so high. So don't get too huffy about the price or having to adjust your calendar by a day or two to find space in the huts. It's actually a pretty easy system.The way most of them work is that you go into a DOC office, tell them what you want to do and they will print you out a sheet saying how many beds are available in each hut for each night. Then you tell them which nights you want to spend in which hut and you're set. If I remember right you can sometimes make a reservation online but I think you have to go to a DOC office to pay for it before you start your hike. Here's what I know about them going from south to north.
Rakiura Track: Reservations for this one are only taken on the island.
This is the least scenic of all the great walks. Kiwis love the place because it's the most unspoiled. I liked it because it was an easy introduction to dealing with DOC and a nice easy warm-up hike. The first and third day are mostly along the coast. The second day cuts across the island to an observation tower with a capacity of one.Kepler Track: This one is a knockout and the only true ridgetop hike in the country. You can even save a few dollars and walk to the trailhead from Te Anau if you don't mind an extra three miles. Reservations need to be made at least a week in advance and you need to pick up and pay for your tickets at the office in Te Anau.
Milford Track: Usually booked solid well in advance so I didn't even bother to ask. A friend of mine from New Zealand actually said he thought the Routeburn Track was better and I also learned that after May 1st reservations are not required. If you're dying of curiosity, it's also fine to day hike from Milford Sound.
Routeburn Track: Outstanding hike with a bit of everything, lakes, passes, falls. The highlight for me was walking along the Serpentine Ridge looking over the Hollyford Valley to the snow capped mountains on the opposite side with a view down to Martin's Bay and the Tasman Sea in the distance.
Heaphy Track: Coastal walks usually aren't my thing but this one was an exception. If it weren't for the sandflies, New Zealand beaches would be among the popular in the world. They are very interesting and very beautiful. The track was just above the beach and well hardened for easy mileage. It's a fairly long stretch along the coast before turning inland to climb over Perry's Saddle and descend to the Aorere River.
Abel Tasman Track: If I'd known then how nice coastal walks could be I might have tried this one but I didn't. There is an alternate inland route that doesn't require reservations although not as scenic. It's also a very popular area for kayaking.
Tongariro Crossing: If I didn't realize how different the two islands of New Zealand were before, this one drove the point home. Where the South Island is formed by plate tectonics the North Island is volcanic. VERY volcanic. And the Tongariro Crossing puts it on display in a big way. Steaming vents, rock formations, several varieties of volcanic sand and luminescent green lakes.
Lake Waikaremoana Track: Didn't bother. It sounded too easy and too crowded and I was having too much fun visiting with friends to notice.
Whanganui Journey: This one isn't actually a walk, it's a river trip. I didn't do it so I don't know much about it. So whether it's included for its scenery or it's cultural significance to the Maori I don't know.