For those wanting to stay on the island.
Visitors stay in houses and rooms across the island since there are no large hotels. The island definitely "fills up" over the holidays and so some planning is needed if you want to stay overnight during the summer. There are various holiday and booking sites such as Waiheke Holiday Homes that give access to a variety of lettings.
The shops and restaurants close quite early and so if you are staying overnight and you tend to eat late in the evening, you'll need to make arrangements to get your food in good time.