Hiking and tourism are big enough and the towns you are likely to stay are small enough that a little advance planning is necessary. You can't just walk into town and expect to find a hotel room or a bed in a hostel. What worked best for me was making sure I had transportation and lodging set for the next town before I set out on a hike. It gave me the right amount of flexibility without leaving me scrambling for a place to stay.
Reservations for some of the Great Walks (Kepler and Routeburn) had to made over a week in advance. Heaphy required a reservation but not as far ahead. Tongariro and Rakiura could basically happen a day in advance or on arrival. So get the Great Walks sorted first, then build your itinerary around that.