They have car rentals at Whakatane (
http://www.whakatane.com/visitor_info.html), and they will always be able to get a taxi for you (if it is like our local airport you will have the same person dealing with unloading the luggage, booking you in, and dealing with rentals etc.). ?If you are staying at a motel, the motel can usually pick you up.
Sounds like you are going to be looked after, anyway! :)