Senior officials considering lending public money to develop Australia's biggest coal mine have known for months of environmental and financial concerns surrounding the proponent Adani, internal emails reveal.

The emails, obtained under freedom of information laws, have fuelled the mine's opponents, who say granting the $900 million loan would pose unacceptable risks to taxpayers and the Commonwealth.

The Adani coal mine would be Australia's largest. Credit:Darren Pateman

Indian mining giant Adani has proposed a $16.5 billion Carmichael coal mine in Queensland's Gallilee Basin, sparking legal challenges and widespread protests.

Australia's big four banks have ruled out funding the project, and Adani has sought a federal government loan to build a railway line from the mine to the Abbot Point coal terminal, near the Great Barrier Reef.