If you want food and drink information from the horses mouth then check out the Woolworths Supermarket Chain's online shop. Here you can find out prices and what's available. Compared to the UK and America the variety of brands is limited but you will be able to find something similar to what you are used to at home. Food and drink produced within New Zealand will be cheap with expected high costs on imported items.
For such a small country New Zealand has an amazing range of quality beers. World class wines are also available at discount prices. As New Zealand is a farming nation fruit, veg meat is of a high quality and cheap. If you are a vegetarian don't worry there are many within New Zealand and all are catered for. Be aware though that in some farmstays they will not cater for veges.
In recent decades New Zealand has earned itself well deserved recognition as a producer of internationally renowned wine. If you enjoy a glass of the good stuff, a trip to one of the many wineries is essential. There are wineries worth a visit all the way from Kaitaia and Kerikeri in the north down to Central Otago. One thing to remember: the legal drinking age in New Zeland is 18 and over.