Queensland taxpayers will help fund a rail line from Abbot Point port to what will be Australia's largest coal mine, with the door open to similar deals with other miners.

Indian company Adani won federal approval earlier this year for the Carmichael mine, which will be unprecedented in scale and represents the opening up the coal-rich Galilee Basin.

Premier Campbell Newman and Indian prime minister Narendra Modi met in Brisbane today to seal the deal, following the weekend's G20 summit.

Mr Newman said similar agreements could be reached with other Galilee Basin mining hopefuls, so the resource-rich region is opened for business.

Queensland would only invest if the companies made bigger investments.

How much taxpayer money would go towards the rail line was not revealed, however the deal could be worth hundreds of millions of dollars.

The money would come from leasing other assets, with taxpayers to have a minority stake in the line.

"The role of the Government, given the financial situation we face these days, would be to make targeted investments to help get something going," Mr Newman said.

"The State Government is prepared to take a short-term, financial stake in the rail, port or other infrastructure needed to open up this region.

"Then within a few years time, exit those investments so the private sector can then get on with it.

"We are now prepared to sign agreements with Galilee Basin proponents who can demonstrate they will meet the majority of the cost of providing this common-user infrastructure."

The Greens said the Liberal National Party (LNP) wanted to sell off Queensland's public assets and give money to mining magnates, to prop up a dying industry that was destroying the reef and climate.

"Not only is this environmentally disastrous, it's economically insane, especially when you're spending the state's public wealth."

Demand for coal drying up, environmentalists says

The project is vehemently opposed by environmental group Coast and Country and Indian group Conservation Action Trust, who have launched a legal challenge in the Land Court.

Indian prime minister Narendra Modi and Queensland Premier Campbell Newman after the deal was signed this morning. ( ABC News: Melinda Howells )

They said the mine, which will export coal for use in Indian power stations, will increase pollution and harm the poorest Indians' standard of living.

The Northern Queensland Conservation Council said Mr Newman was backing a loser.

Coordinator Wendy Tubman said demand for coal is drying up.

"The banks are walking away from the industry, the world is transitioning to renewable energy, and yet our government is proposing to pour hundreds of millions of dollars into this dying industry," she said.

The Queensland Resources Council said the state investment in the mine showed confidence in the state's coal industry.

"We believe that this announcement today will attract a lot of interest to the project," QRC chief executive Michael Roche said.

Adani Australia chief executive Jeyakumar Janakaraj echoed Mr Roche's sentiment.

"It gives confidence to investors who considering their long term plans in the basin that the state is committed to the best possible infrastructure being in place to support the further development of the Galilee, and the jobs and exports that will provide," he said.

Queensland to pocket billions from mine

The project is expected to inject $500 million into Queensland's economy during construction and $3 billion at full export capacity.

The $16.5 billion development will produce 60 million tonnes of coal a year, with an estimated lifespan of 60 years.

The mine will be 60 kilometres long, spanning an area seven times the size of Sydney harbour.

The 388-kilometre rail line will run to a new terminal at Abbot Point Port near Bowen in north Queensland.

Work is due to begin next year.