Volcanic Air Safaris do them in Rotovegas,
(07 348 9984,
http://www.volcanicair.co.nz/ )
Also I think you can do them with the guy in Taupo
http://www.tauposfloatplane.co.nz/
Somewhere down Invercargill was used to do them too, not sure who. Not sure if the Wakatipu aero club in Queenstown still does them.