The Royal Forest & Bird Society of New Zealand is against a proposed irrigation scheme in the lower South Island.
Farmers are seeking water rights to irrigate more than 27,000 hectares of the MacKenzie Basin and the upper Waitaki valley. They say the area is turning into a desert.
Forest & Bird South Island conservation manager Chris Todd told the Sunday programme on Radio New Zealand National that the scheme would be unsustainable.
He said it involved the "wholesale clearance" of rare ecosystems and would also divert water from hydro schemes. There would also be nitrogen run-off from more intensive land use.
However, local Federated Farmers chairman John Murray says all but three or four of the applicants have excluded dairying as a use for the water.
An independent panel of Commissioners will hear submissions over 10 weeks from September.
