The state Labor government offered a "royalties holiday' to the Indian company, which could cost taxpayers $320 million, the ABC reports.
Under the agreement, expected to be signed as early as this week, Adani would pay just $2 million annually in royalties for the first seven years of operation of its $21 billion mine.
State Treasurer Curtis Pitt on Thursday rejected suggestions a special arrangement had been hatched after Premier Annastacia Palaszczuk failed to confirm or deny the story.
"No taxpayer funds will be provided to Adani to support the construction of a rail line and Adani will pay every cent of royalties owed to the state," Mr Pitt said.
The news follows the release of a Climate Council report, which warned of serious health risks posed by the mine, prompting protests by non-profit organisation Doctors for the Environment Australia.
The report shows that if the Galilee Basin mine were a country, it would rank among the top 15 worst emitting nations in the world, making up 130 percent of Australia's current annual pollution levels from all sources.
"It's an absolute no brainer on any scientific calculation, you've got to leave that coal in the ground if you're serious about tackling climate change," says climate scientist Professor Will Steffen.
Dr Peter Brooks, a member of Doctors for the Environment Australia, says the biggest effect of climate change is on health.
"One of the problems about coal is that not only are we affecting climate change, but it’s the particulate matter – these very small particles that get into the air, get ingested, get into the lungs," Dr Brooks says.
"They drive heart disease, they drive lung disease. We’ve just had an epidemic of coal miner’s lung in Queensland… what’s the Carmichael mine going to do to that sort of thing?"
Traditional owners return to court
Meanwhile, the Wangan and Jagalingou Family Council returned to court today, seeking to invalidate Adani's Indigenous Land Use Agreement.
"The position now quite clearly is that if the parliament makes a decision to pass the amendment bill, it will bail Adani out of a difficult situation," says Colin Hardie, lawyer for the Wangan and Jagalingou Family Council.
"’Adani at the moment has nowhere to go without the assistance of the federal parliament."
Adani's Indigenous Land Use Agreement is one of more than a hundred called into question following the McGlade court ruling in Western Australia, which found agreements must be signed off by all registered native title claimants, rather than just a majority.
The federal government is seeking to reverse that ruling through amendments to the Native Title Act, which will be presented to the senate next month.
'Those amendments are specifically about providing Adani free passageway to essentially destroy our country forever.'
"Those amendments are not about Native Title Reform," says Wangan and Jagalingou youth spokesperson Murrawah Johnson.
"Those amendments are specifically about providing Adani free passageway to essentially destroy our country forever, who we are as a unique people from that country. We won't allow it."