About 100 kayakers took to the waters of the Mokihinui River in Westland on Sunday to protest against plans to dam the river.
The dam is planned by the power company, Meridian Energy, but the protesters says the river is one of the country's last great wilderness areas and should be preserved as it is.
Forest and Bird spokesperson Debs Martin says the event was months in the planning, and the kayakers had to be brought in by helicopter.
She says the turnout shows the strong support for preserving the river, which she believes should remain in the conservation estate.
Ms Martin says the protesters will continue to do all they can to persuade the Government to reject Meridian's plans.