The Department of Conservation is opposing Meridian Energy's proposed Mokihinui hydro-dam, north of Westport.
At a resource consent hearing for the project in Westport today, DOC's submission said while it supported renewable energy production, it did not believe the Mokihinui River was a suitable location.
The Westport News reported the submission argued the dam would destroy a lot of native bush and would threaten at least 18 different endangered species.
Public access would be more difficult and historical sites within the river catchment would be damaged and destroyed.
DOC said the proposed route to get the electricity out would cause too much harm, both during construction and once it was working.
Meridian wants to build an 85m high dam, 11km upstream from the river mouth, creating a 14km lake.
The dam would produce enough electricity for 45,000 to 51,000 households.
Meridian have argued that the Mokihinui was the only river in the district suitable for large-scale hydro-generation. Other main rivers in the area had protected status or insufficient flow.
The Mokihinui did not have elevated protection status. However, one area of the south branch had been identified as a priority site due to the presence of native blue ducks.
The hearings are expected to be completed by October 31.
If the dam passes the consent stage, it will take three years to build.