The scale of water demand from Adani's giant coal mine in Queensland appears to be larger than expected, as the company seeks approval for a large, additional flood-harvesting dam.

As Fairfax Media revealed, the company's proposed Carmichael coal mine in the Galilee Basin was last week controversially granted uncapped access to the groundwater in a licence that expired in 2077.

However, just how much surface water the mine will need remains unclear. Its "external water demand ... is in the order of 12 gigalitres a year", according to its supplementary environmental impact statement (SEIS).

Documents obtained under freedom of information, however, suggest the thirst for water at what would be Australia's biggest coal mine may be even larger.