There is no reticulated water on Great Barrier Island. Bottled water is available at many shops, but that can be costly. Due to the presence of Giardia in waterways you should boil all collected water or use a filter or purification tablets.
This is not so much a warning as a heads up. Be sure to have enough cash BEFORE boarding a boat or plane off the mainland. There are NO banks or ATM (Automatic Teller Machines) on Great Barrier Island. Some shops have EFTPOS (Electronic Fund Transfer at Point of Sale) machines and except credit cards. But it's still a good idea to have cash on hand ,so you don't come up short !
TRIVIA: New Zealand was the 1st country in the world to have EFPOS. This new technology as well as many other new products and inventions were tested in New Zealand before being marketed worldwide.