from my experience, you can pretty much use credit card (almost) everywhere you go in new zealand. we did a road trip around the country and never had a problem. even at smaller towns, and i remember we went to this salmon farm in the middle of nowhere and they have credit card facility.
tho, please note that atm is not prevalent esp in smaller cities, you might want to withdrawal cash with your bank atm card in advance at places where you see atms. you can't use your atm card to make eftpos (like EPS in HK) over in NZ cos your card doesn't run on the eftpos system (unless if its an aussie/kiwi bank card that is) so otherwise, stick to your credit card for bigger purchases.
internet, wireless or lan, are almost never free in B&Bs and 4/5-stars hotels. not sure about lodges or backpackers hotels.
have a great time. i've always loved nz.