A five million hectare slice of Western Australia's Kimberley region will become the country's largest national park after the State Government struck a deal forever banning mining in the iconic Mitchell Plateau.

After extensive negotiations, a 45-year state agreement that gave Rio Tinto rights to mine bauxite and Alcoa the right to refine aluminium on the Mitchell Plateau has been cancelled.

No further mining or exploration will be permitted in the 175,000 hectare area, which will be included in the new five million hectare Kimberley National Park which includes a network of land and marine parks.

Premier Colin Barnett said thanks to the agreement, the "extraordinary" landscape would be preserved, delivering a major conservation outcome.

"I genuinely believe that this is the most significant conservation achievement in Western Australian history," Mr Barnett said.

Legislation will be introduced into Parliament this week to cancel the state agreement.

Mr Barnett expected it would have bipartisan support.

The park will incorporate two million hectares of land in the Kimberley, taking in the current Prince Regent, Mitchell River and Lawley River national parks.

Park 'must be seen to be understood'

Termination of the state agreement with Rio Tinto and Alcoa allows the Mitchell Plateau and Mitchell Falls to form the centrepiece of the new park.

Rio Tinto chief executive Sam Walsh said seeing the area firsthand convinced him it should never be mined.

"As I toured around, I was visiting as a tourist, not as an employee of Rio Tinto, and I met people and I said 'do you think this is going to be developed'," he said.

"People said this will never be developed and quite frankly I believed them.

"To fully understand you need to go there and you need to physically see it.

"It is iconic. There's no question in my mind that this is an iconic area that needs to be preserved for future generations."

Mr Walsh said even if aluminium refining had been viable, the company would not have mined the bauxite in the region because of the inevitable impact on the environment.

"Bauxite mining is on a very, very broad base," he said.

"Generally, bauxite deposits are 15 metres deep but across a wide expanse.

"You could not mine this area without impacting on the environmental and heritage aspects."

In negotiations with the Government, Rio Tinto secured an agreement to ensure that if it relinquished its rights, the agreement would be terminated for all mining.

The company will contribute $750,000 to rehabilitation work in areas where some drilling was conducted.

Mr Barnett said the company's decision to relinquish mining rights was a great contribution to preserving the Kimberley.

"That was an extraordinarily generous act by Rio Tinto, putting the interests of conservation ahead of their interest of ownership in that mineral deposit," he said.

Mr Barnett said he expected the complex work of establishing the park's boundaries would begin soon, but he had not set a deadline for when the park will be formally in place.