I have been by helicopter a couple of times, but many years ago, as my father used to own one of the companies that flies there (Vulcan Helicopters). He has since sold the business and has no interest in it.
They(Vulcan) fly out of Whakatane, you can also take the trip from Rotorua through a different company (sorry do not have details - google).
While it is probably more expensive, you may find it more convenient to depart from Rotorua depending on the rest of your plans.
I'm not sure if there are any other (helicopter) operators as the island is privately owned and they restrict the granting of licenses to land there.
Either way it is quite a unique trip and worth doing.