It is quite possible to get by without any cash - as has already been mentioned, NZ has the most extensive EFTPoS network I've experienced and small transactions aren't a problem. It might be worth having NZ$100 in cash for emergencies - I usually do this, but also usually end up with cash left over! You might not need cash in Auckland or Queenstown (except for things like bus fares), but if you venture into the country, it might be useful for things like wine tasting fees (usually about $5, though you could probably pay with card) or a snack at a small shop.