Adani Australia says the loan it is seeking from the Federal Government for its proposed Carmichael coal project could be less than half of the reported $1 billion.

"It may be more than $500 million, it may be less than $500 million," company spokesman Ron Watson told 7.30.

"It all depends on our financial arrangements, which we've yet to finalise."

Adani Australia applied for funding under the Federal Government's Northern Australia Infrastructure Facility (NAIF) in December, to help with the construction of a 380-kilometre railway line linking its proposed central Queensland coal mine to the coast.

It was widely reported at the time that the then-minister for Northern Australia, Senator Matt Canavan, had confirmed the loan application was for $1 billion.

But this was called into question in a recent interview Adani Australia's chief executive Jeyakumar Janakaraj gave to Queensland's Courier Mail.

"Even I don't know the number," he was quoted as saying.

"It could be anything."

Adani says it's awaiting decision on NAIF funding

The Indian-owned company also said there may be a three-month delay to finalising its finances.

Adani originally said it wanted to confirm all finances by the end of this year.

But Mr Watson said that deadline might have had to be put back to the first quarter of 2018, while it awaited a decision on NAIF funding.

"The timing is out of our hands," he said.

"However we still believe we can meet our schedule of having the first shipment of coal in 2020."

NAIF confirmed last month that it had selected five projects out of 124 loan applications for due diligence, and the first loans should be finalised by the end of September.

NAIF did not release details of specific projects, citing commercial-in-confidence laws.