Australia's biggest ever coal mine will go ahead, according to Indian billionaire Gautam Adani, whose company has made the final investment decision to "green light" the controversial Carmichael project.

In a statement released on Tuesday, Adani said it had signed letters of award for design, construction, operations, supply of materials and professional services.

The company said the biggest contract is with Downer Mining for the construction and operation of the Carmichael mine located in Queensland's Galilee Basin.

DARREN PATEMAN/FAIRFAX MEDIA The company has plans to mine 2.3 billion tonnes of coal from Carmichael.

Construction of the mine, which could eventually entail six open cut pits and five underground mines over an area 30 kilometres long, has been bitterly opposed by environmental groups.

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The company has said it plans to mine 2.3 billion tonnes of coal over the mine's 60 year lifespan.

AMIT DAVE/REUTERS Indian billionaire Gautam Adani says his company is making the biggest single investment by an Indian corporation in Australia.

Adani said: "This is an historic day for Adani, an historic day for regional Queensland, and an historic day for Indian investment in Australia.

"This is the largest single investment by an Indian corporation in Australia, and I believe others will follow with investments and trade deals.

"We have been challenged by activists in the courts, in inner city streets, and even outside banks that have not even been approached to finance the project. We are still facing activists. But we are committed to this project."

But the Stop Adani Alliance, made up of 20 environment groups, immediately called the Adani announcement a "con" because the company has been unable to secure the finance it needs to actually start mining.

There was no detail in Adani's statement about finance.

Stop Adani Alliance spokesman Geoff Cousins, who is also president of the Australian Conservation Foundation, said it was "groundhog day" because the company had announced other "investment decisions" over the past six years.

"Adani is yet to lure any financial institutions willing to bankroll the project, which is proving hugely unpopular with the Australian public. Without this support and public subsidies, the heavily leveraged balance sheet of Adani Enterprises leaves no internal capacity to fund this A$5 billion project proposal," Cousins said.

"If the federal government hands Adani A$1 billion of public money for this destructive mine that will destroy our reef, we will consider all avenues, including legal action, to stop it."