If you are going to New Zealand to hike the multi-day tracks called the Great Walks, this is a pretty good guide to them, but is somewhat dated (over 4 years old). What really surprised us on our recent trip is that for the most part you can't do a day hike on many of the famous Great Walks such as Milford, Kepler, or Heaphy. The trails are designed to be one-way multi-day hike trails with a designated start and end spot for backpackers who want to camp or stay in the huts.
There are a couple of locally written hiking books that have good information on 2-8 hour day hikes in New Zealand that let you sample some of the Great Walks that are not one-way routes. One of the most useful is "Day Walks in New Zealand" by Shaun Barnett. It has 100 day treks on both the north and south island with some amazing two dimensional maps and beautiful photos. We used the book on a two week trip to the South Island and found some wonderful hikes from the book in the Milford Sound area that let you stay at nice hotels and lodges and still experience the back country and beauty of New Zealand. For example, the book highlights the hike to Lake Marian near the Hollyford Track and the day hike to Key Summit which is part of the Routeburn Track. It also has wonderful short hikes in Abel Tasman National Park using the water taxi services, several hikes in Mt Cook National Park, and multiple tramps in Mount Aspiring Park as well. The book is produced in New Zealand but is listed on Amazon so you should be able to buy it before you leave.
Another great resource for shorter hikes is the New Zealand Department of Conservation website: [...] It has a lot of useful planning information to supplement any books you purchase on Amazon to help in your trip planning.