Forest & Bird says a report on the proposed Ruataniwha irrigation dam reinforces the "well-established principle" that rivers must not be allowed to get so polluted that they cannot sustain life.

The Board of Inquiry into the Tukituki Catchment Proposal's final report has given the dam a consent, on the condition that properties irrigated by the water storage scheme meet defined nutrient leaching rates, so that nitrogen and phosphorus limits are met downstream in the Tukituki River.

Forest & Bird said if built, the dam was expected to result in an intensification of agriculture in the Hawke's Bay, which in turn would result in more nitrogen and phosphorous reaching the Tukituki River.

"This decision is a big win for the health of New Zealand's lakes and rivers.

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"The report sets a strong precedent in regards to the freshwater quality standards that will implement the government's National Policy Statement on Freshwater Management," Forest & Bird advocacy manager Kevin Hackwell said.

"The National Policy Statement on Freshwater Management requires the quality of lake and river water to be maintained, or improved, if it falls below certain limits.

"The idea that the Tukituki catchment could be allowed to become toxic was rejected by the Board of Inquiry, which after hearing weeks of evidence accepted that if we are to have healthy ecosystems, nitrogen needs to be controlled to low levels. The government should pay attention to the findings of the board it appointed.

"The board's findings are also consistent with the recommendations of the Land and Water Forum, a multi-stakeholder body that includes farmers, iwi, and environmentalists.

"The forum concluded that our national standards must not allow freshwater to get so polluted that it cannot sustain life.

"The forum also put a lot of work into making recommendations on how to manage water quality within these limits, and how to clean up polluted waterways if those limits are exceeded."

He said the society was still carefully considering the other provisions of the Board of Inquiry's final decision on the plan change and consent conditions.

The board of inquiry's final decision was also welcomed by the Environmental Defence Society, which opposes the dam.

EDS chairman Gary Taylor earlier said the society was still assessing the implications of the changes the board had made to the way nitrogen would be measured but the future of the dam was "questionable"

"The risks of proceeding are high now that water quality must be maintained or improved in the Tukituki River," he said.