An aerial view of irrigation circles near Lake Benmore in the Mackenzie Basin.

A handful of farms want to further intensify the Mackenzie Basin, described as a "stronghold" for rare and threatened species that exist nowhere else in the world.

Six of the eight farms that make up the Benmore Irrigation Company (BIC) have applied to nearly double their irrigable area.

Opponents say it would be disastrous for a unique landscape home to more than a hundred species of native plants and birds.

LINZ/SUPPLIED Irrigation circles already cover large parts of the Mackenzie Basin.

It would involve clearing at least 1250 hectares of indigenous biodiversity — an area around eight times the size of Hagley Park — to make way for pivot irrigators and farmland.

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The impact of intensive farming in the basin is clear from above, with large, alien-like irrigation circles visible along the state highway between Twizel and Omarama.

DEAN KOZANIC/FAIRFAX NZ An irrigator in the Mackenzie Basin.

Landscape architect Di Lucas said the proposal would further change a landscape that had taken millennia to develop.

"Naturalness will be diminished, native vegetation will be lost, and visual quality will lessen as a result of the proposed irrigation," she said in her written submission.

"The adverse landscape effects... are highly significant, not able to be remedied or mitigated, and [the] proposal in its entirety is inappropriate."

DI LUCAS/SUPPLIED The orange outlined areas are proposed for irrigation. They overlap Outstanding Natural Features marked in yellow.

The company had not applied to use more water, but to use its existing allowance more efficiently.

More irrigation meant farms could eradicate pests and bring economic benefits into the community, the company said.

The effects would be "less than minimal," which was why it asked for its application not to be notified to the public.

It was notified, however, and found opposition from Environment Canterbury (ECan), the Department of Conservation (DOC), and environmental groups such as Forest & Bird and the Mackenzie Guardians.

Two of the 30 submitters supported the plan, one of which was Federated Farmers.

Ecologist Nicholas Head, on behalf of the Department of Conservation, said the company had "seriously understated" the impact their plan would have.

The basin was a "stronghold" for some of the country's "most distinctive, rare and threatened ecosystems and species," he said, which had been degraded by years of land-use change.

"This will result in the permanent loss of irreplaceable, naturally rare ecosystems," he said in his written submission.

"[It] highlights the very real risk that ongoing land-use change... will result in extensive and permanent losses of indigenous ecosystems and species that are still present and which exist almost nowhere else."

ECan opposed the application due to "significant uncertainty" about the effect on water quality and ecosystems.

ECan land resource scientist Dr Philip Grove said it would likely impact threatened native birds such as banded dotterel, which have a large population in the basin.

Forest & Bird opposed it for similar reasons, saying it would be "another nail in the coffin" for several bird and plant species.

The application is being considered by an independent hearings panel, which heard arguments last week in Omarama.