Environmentalists are happy with a High Court ruling on the Ruataniwha Dam, which offers some reassurance on water quality standards.

A judge has found the Board of Inquiry into the dam set up a system which wouldn't achieve national standards.

Forest and Bird's Kevin Hackwell is pleased the judge has forced the Board to toughen its rules, to limit the amount of nitrogen which can leach into the water from farms.

He says the decision sends a strong message to regional councils.