The Department of Conservation says a hydro dam proposed for Buller would involve the most extensive flooding of protected lands and ecosystems for a power scheme in New Zealand.
The project involves a dam 85 metres high.
DoC is calling 15 witnesses at a resource consent hearing in Westport, opposing Meridian Energy's scheme on the Mokihinui River.
Meridian Energy says the project is vital for West Coast power needs and its effects would be minor.
DoC says Meridian has refused to consider alternative sites for hydro generation on the West Coast.