I do not know of any agencies. I was on a tour on Nuku Hiva as part of the Aranui cruise. When cruise ships arrive about all locals who own 4WD's come to the wharf in their vehicles and pick up the visitors. That's about 50 vehicles... At least when the Aranui arrives all vehicles are booked by the cruise company, there would not be a chance to find somebody else who could take you on a tour.
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