I asked this question when planning our trip to the South Island of NZ a few months ago and just wanted to give my experience.
We just returned last weekend and we never had a hard time finding accomodations anywhere on the South Island. In fact, every town we stayed in had many vacancies. We were apprehensive about not planning any tours/hotel reservations and had no problems at all.

I suppose if there was a specific hotel you had to stay at, then it may have been different. We weren't very picky with the hotels.

We stayed at:
The Manor on Bealey, Christchurch
Anchor Inn, Kaikoura
Grampian Villa, Nelson
Westport (don't remember the name--we were tired of driving so stopped for the night)
Fox Glacier Lodge, Fox Glacier
Bella Vista Motel, Lake Wanaka
Luxmore Hotel, Te Anau
Doubtful Sound Cruise,Real Journeys
Scenic Circle, Queenstown
Coronation Lodge, Queenstown
Aoraki Alpine Lodge, Mt. Cook
Copthorne Hotel, Christchurch

All of these were either booked the same day through i-site or we just drove up and asked if there was availability. They are very good about putting vacancy/no vacancy signs up.

All tours were booked upon arrival in the different cities also. We had no problems. Maybe we just got lucky. Either way, this is the way to go. No stress to hurry up and get the the next town because you have a hotel reserved. Stay as long as you want. It was awesome!!

Thanks for all the help in planning our trip

Some of it was luck no doubt.

Also if you were there during a spell of fine weather (which I think was true), then the backpackers are not competing with you for last minute hotel rooms.

Stopping early in the day is also helpful for booking same day rooms.

Indeed, you can (?almost) always find a place to stay. But sometimes they can be less satisfactory, especially in the small locations.

Consider Fox Glacier Hotel. Didn't get good reviews in Tripadvisor:
http://tinyurl.com/3799ok

How was it?

We stayed at Fox Glacier Lodge--different from Fox Glacier Hotel.

The lodge was new and very clean, good location. We had gotten the free standing villa and were very happy. They provide bread, cereal, fruit, yogurt, juice, milk, etc. in your mini kitchen for you to have for breakfast the next day (included in the price). Also, they provide a guest laundry for 3NZ. This was very helpful since alot of our clothes were wet/dirty from riding the motorcycle in the rain for a day.

As far as the weather goes, we were told that they were having their coldest summer in 62 years. It was raining part of 3 days while we were there and still had no trouble finding accomodations.

We weren't being very picky though. For us, the room was just a place to sleep and clean up before moving on.

If it only rained part of three days you had very good weather. I was there for nearly a month in Nov-Dec and I think it was sunny for part of three days!

In general I agree with you, it is nice not to be on a schedule. However it is not so nice to arrive in town and find that nothing is available in your price range. This can happen if you show up AFTER the backpacker busses have pulled into town. If you are flexible and don't mind staying outside of town, you can generally find something. Eg Omarama or Twizel, instead of Mt. Cook proper. The towns with the tightest accommodations seem to be the Glaciers, Mt. Cook (if you want something less expensive), and Christchurch if there is a special event or convention.

Sorry I mixed up Fox Glacier Lodge and Hotel.

No problem