Environmental organisations will challenge a board of inquiry's final decision on the proposed Ruataniwha water storage scheme in the High Court.
Both Fish and Game and Forest and Bird have confirmed they are challenging the decision on points of law.
Forest and Bird's advocacy manager, Kevin Hackwell, said the board introduced sound limits on nutrients leaching into the Tukituki River when it approved resource consents for the scheme.
However, he said it failed to say how these limits could be achieved as required under the National Policy Statement on Freshwater Management.
The proposed $600 million water storage scheme could irrigate 25,000 hectares of Hawke's Bay farmland.
The Hawke's Bay Regional Council's investment company HBRIC and the Regional Council itself, both announced in July they would not be appealing against the decision.
The project still has some major hurdles to overcome, including finding enough investors and customers.