Indian energy giant Adani says a $900 million federal loan to build a railway line to its proposed coal mine in central Queensland is now critical to the project, despite previously insisting the company did not need the loan.

Company spokesman Ron Watson has told ABC TV's 7.30 program that the position had changed over the past few months.

"All commercial elements of the financial equation are regularly reviewed," he said.

"They have moved from where they were some months ago."

The 389-kilometre railway line would link Adani's proposed $21 billion Carmichael coal mine in Queensland's Galilee Basin to the port at Abbot Point, north of Bowen.

Last December, Adani applied for a loan to help build the line through the North Australian Infrastructure Facility (NAIF), which was set up to encourage private sector investment in the region.

Under its funding rules, NAIF can only support projects that are "unlikely to proceed of will only proceed at a much later date, or with a limited scope, without financial assistance".

Adani said a final investment decision on the Carmichael Mine and rail project was expected within the next two to three weeks, opening the way for construction to start in August.

The spokesman said the details of financing were expected to be finalised by the end of the year.

The proposed railway line would be open to other users, with the Minister for Northern Australia, senator Matt Canavan, arguing it had the potential to spur at least another three mining ventures in the basin.

"The level of wealth that could be unlocked is comparable to that of the Hunter Valley or the Bowen Basin," he said.

Federal Opposition Leader Bill Shorten supports the mine, which was expected to generate thousands of jobs, but was opposed to the granting of a Commonwealth loan.

Green groups are opposed to the mine, arguing carbon emitting fossil fuels like coal should be phased out in order to tackle climate change.