Commissioners who gave the green light to an 80m-high hydro dam in the Mokihinui Gorge north of Westport were overwhelmed by the gorge's natural beauty, but said there were other equally beautiful gorges in the area.

John Lumsden, a civil engineer, and West Coast councillor Terry Archer outvoted fellow hearings commissioner Greg Ryder to say yes to the dam - the largest flooding of conservation land for hydro power ever proposed in New Zealand.

Explaining why they gave consent, the pair said: "If the Mokihinui River were to be considered in isolation, it would be difficult not to form the view that to build a dam and flood to gorge would be a travesty.

"When we walked through several sections ... we could not help but be overwhelmed by its natural beauty."

But they said that beauty was not unique on the West Coast, or even in the Buller District.

The renewable electricity the dam generated would give locals a more reliable power supply and take a load off fossil-fuelled power stations during dry years, they said.

The dam would flood 337ha of the gorge to create a 14km-long, 80m-deep artificial lake about 3km upstream from the small town of Seddonville.

The Mokihinui River is part of a largely untouched catchment that flows from the Kahurangi National Park through stands of rimu, rata and matai to the sea.

Mr Ryder, an ecology consultant, voted against the dam, saying it was more important to leave the area unspoiled.

The three commissioners agreed that losing 280ha of native forest and the homes of native ducks and snails was a serious downside.

State-owned power company Meridian has offered to partially make up for it by clearing a 6000ha sanctuary for native species in the lower reaches of the river.

A field officer with the Forest and Bird conservation group, Debs Martin, said that could not compensate for the loss of the river and the group was considering an appeal.

The Minister of Conservation, Kate Wilkinson, would have to approve the dam as it is on land managed by her department.

Ms Martin said she could not see how the minister could say yes without questioning the integrity of the 16 Department of Conservation experts who gave evidence opposing the dam.

"If this goes ahead it will be the largest inundation of conservation land ever seen in this country [for hydro]," said Ms Martin. "It is pristine forest with tall rimu and rata."

Buller Mayor Pat McManus is happy because he says construction will employ more than 300 people over three years, dropping to six once the dam is running.

But whitewater kayakers, trampers and fishermen - including Revenue Minister United Future leader Peter Dunne - are furious.

Freshwater ecologist Mike Joy told the hearing that native fish were likely to die off if the river was dammed.

Ms Martin said the dam would be the equivalent of a 20-storey building blocking the river.

"It will be like turning a tap on full bore and then turning a tap on trickling [below the dam]."