An area of unspoilt conservation land that was the centre of a major environmental row will now become the largest addition to a national park in New Zealand's history.

A total of 64,400 ha in the Mokihinui River catchment on the West Coast north of Westport, including 15km of riverbed, is being added to Kahurangi National Park.

Conservation Minister Eugenie Sage said national park status would ensure stronger protection of the Mokihinui area's significant cultural, ecological, historic and recreational values.

The Mokihinui River was not included in the national park when it was established in 1994 in part because of opposition from pro-development interests.

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Instead the area remained stewardship land – conservation land which is under-protected and frequently targeted for development.

A dam scheme proposed by Meridian Energy for the Mokihinui River in 2007 attracted heavy opposition because of its environmental impacts.

The 80m-high hydroelectric dam would have drowned a stretch of river and forest within a 14km-long reservoir.

"It would have flooded the Mokihinui Gorge and inundated beech-podocarp forests and significant habitats of threatened plants and wildlife such as whio/blue duck, kaka, bats and giant land snails," Sage said.

Forest & Bird and other groups lodged appeals against Meridian's resource consents and campaigned to stop the dam until Meridian cancelled the project in 2012.

That decision was followed by a groundswell of support for giving national park status to the Mokihinui catchment and adding these lands to the park.

"A big thanks to the many New Zealanders and the Department of Conservation who spoke up for the river, its gorge, dramatic landscapes, beech-podocarp forests and set out the reasons they deserved protection from a hydro dam," Sage said.

"Today's announcement is only possible because of that work and advocacy. It is why our Government can now give the Mokihinui Gorge, and the surrounding lands, forests, and mountains the strong protection that comes with being part of a national park.

"Protecting these lands means generations to come will be able to enjoy these beautiful natural landscapes, ride along the Old Ghost Road track alongside the Mokihinui River, and see and hear birds like whio and kaka."

Kahurangi National Park is the country's second largest national park. Photo / Nigel Boniface
Kahurangi National Park is the country's second largest national park. Photo / Nigel Boniface

Forest & Bird chief executive Kevin Hague said it was "fantastic" to see the land getting protection.

"If it had been properly protected in the first place, Forest & Bird, along with several other organisations and countless volunteers, would not have had to spend years fighting to protect the Mōkihinui River from schemes such as these," he said.

"With New Zealand's environment in crisis, and the impacts of climate change expected to increase, now more than ever our public conservation land is needed for its real purpose, conservation."

The Mokihinui land - equivalent to half the size of Auckland - become part of Kahurangi National Park on April 11.

The land being added to the national park stretches inland through the length of the Mokihinui River catchment.

It includes about 15km of the Mokihinui riverbed, all of the Mokihinui Forks Ecological Area, a large part of the Lyell Range-Radiant Range conservation area and a small part of what remains of North West Nelson Forest Park, most of which became Kahurangi National Park when it was established in 1996.

The addition connects with the southwest boundary of Kahurangi National Park.

The Mokihinui addition to Kahurangi National Park is equivalent in size to Abel Tasman and Paparoa National Parks combined and is twice the size of Egmont National Park.

Kahurangi is the country's second largest park and with the addition of the Mokihinui land, it will increase in size by 14 per cent to 517,335ha. Fiordland National Park, at more than 1,230,000ha, is the largest national park.