This was easily the best hike we did in NZ.  It took us three days and you climb right up high in the mountains where you get the most amazing views.  It's one of the Great Walks so you need to book it in advance but it isn't as popular as the Milford walk so it's usually possible to get spots.  You sleep in DOC huts along the way which consist of enormous shared dorm rooms and a large communal kitchen.  Perfectly suited to hiking.  We thoroughly enjoyed all of the hikes we did in the South Island and I definitely want to try others but to date, this one is my favourite.