Hi,
If u are buying stuff from the supermarket, try and buy when u are in the bigger towns, cos the smaller towns do not have big supermarkets and the selection is very limited. For e.g made the mistake of driving to Lake Tekapo and thinking of buying groceries when we get there, but realised that they had only a really small supermarket which had very limited selection. And there were no big supermarkets nearby. But no problems with the bigger towns like Dunedin and Queenstown, definitely will have supermarkets.
When i went NZ, brought instant noodles, soy sauce, steak seasoning, chilli powder, instant soup etc. Cooked almost everyday while i was there, more fun. U can also buy takeaway fish and chip, very nice and quite cheap. When u are there, u will find that the restaurant food is quite expensive, even when u go to the supermarket, some of the food is more expensive than Singapore, which is quite surprising. E.g when u buy steak in supermarkets, can be more expensive than when u buy in singapore. But if u buy from the butcher, it is quite reasonable.