Beware of the taxi touts at the wharf. Legally the drivers cannot solicit for business if they do under no circumstances go with them as the ones that use these tactics (usually the ones in vans) have no meters and will rip you off.
They may claim to be longstanding locals with the best rates but you have been warned use a cab with a meter or catch the bus.
This is more of a warning than a danger but a large section of the island is privately held land that has a public road running through it mainly to allow access to Stoney Batter. When you drive down this part of the island you will notice gorgeous beaches and inlets and possibly areas you might be tempted to walk across. Please refrain from doing so and look at for these signs.
If you do rent one of those racey little sports cars or go-anywhere 4WD's from Waiheke Auto Rentals, just remember the Police are well equipped on the island and you won't get away with crazy behaviour.
Good luck anyway....