I think you will probably find the towns there a lot different to the pictures you have in your mind, we very surprised to find Westport which has a population of less that 4000 which is smaller than the village we live in (2 garages, 2 restaurants, 2 shops, 3 pubs), but it had dozens of shops, garages, campsites etc, it felt like a town many times larger. Same with Christchurch, the suburbs seem to go on forever. Although the towns are a lot smaller, there have much more facilities that a comparable British town.
Even a "town" as small as 400 you could expect a shop and a pub. Haast township has 300 and had a motel, shop, fish & chip shop, garage and probably a lot more that we didn't notice.