"But it is eminently sensible to increase the net conservation value by trading away higher value for lower value conservation land."
Conservation Minister Maggie Barry echoed the Prime Minister.
"The appeal to the Supreme Court was not about whether the Ruataniwha Dam should go ahead - it was about obtaining clarity after differing and split decisions from the High Court and the Court of Appeal, she said.
"For the past 30 years we all believed that the legislation allowed the swap of a low value piece of conservation land for a piece of land with higher conservation values.
"We will now look at changing the law to ensure we can continue to improve conservation outcomes by having the ability to make land swaps where the outcome would be a win for conservation."
Three of the five judges assigned to the case found the Minister of Conservation acted illegally by trying to make the land available for exchange to the Hawke's Bay Regional Investment Company Ltd (HBRIC) for the $330 million project.
It marks the end of a lengthy and controversial battle through the courts that began in 2015 when DoC director general Lou Sanson approved the exchange of 22ha of the Ruahine Forest Park for 170ha of land HBRIC would potentially buy from Smedley Station.
Forest and Bird challenged this in the High Court and then the Court of Appeal.
In August last year, the Court of Appeal ruled the land swap deal was illegal, after which the Conservation Minister and HBRIC took the case to the Supreme Court, where it was heard in February this year.
Forest & Bird welcomed the announcement as good news for Ruahine and all forest parks around the country, said chief executive Kevin Hague.
"The Government went all the way to the Supreme Court to allow the downgrade and exchange of part of Ruahine Forest Park, which would have led to the destruction of that land. The Supreme Court has confirmed that our forest parks belongs to the people of New Zealand and are protected by the Conservation Act."
HBRIC chief executive Blair O'Keefe said the decision was a setback for the scheme, but would be considered including discussing the implications with all key stakeholders.
HBRIC director Sam Robinson said talk of acquiring land through the Public Works Act had not been discussed seriously but that could be one of the options to consider moving forward.
"We need to take further advice - we have a board meeting later this month and will know more after that."
The Hawke's Bay Regional Council had planned to invest $80 million in the project if it went ahead, $20 million of which had already been spent.
It recently, however, put several further conditions in place for its investment, one being that the land exchange issue be resolved before going ahead with the scheme.
HBRC chairman Rex Graham said he wasn't surprised by the decision and that the council and HBRIC would now need to consider the options going forward.
"The council is very split on this - some are passionate for the dam, others are passionately against and some are in the middle - there's a wide range of viewpoints but we all have to live with this decision."
When asked his thoughts on the council using the Public Works Act to acquire the land, he said he wasn't in favour.
"I know that's an option but I would strongly oppose council going that way - some might want to but I am pretty certain most wouldn't - I think it would be irresponsible of a regional council to do that."