Another potential stumbling block looms for Adani and plans to expand its Abbot Point coal port after conservationists flagged “material deficiencies” in an application for commonwealth approval before the environment minister, Greg Hunt.

The North Queensland Conservation Council (NQCC) says it has uncovered secret plans for a tugboat harbour which its legal team indicated could undermine the basis for Hunt’s decision, due next week.

The NQCC says the proposed harbour would heighten the cumulative environmental impacts of the port expansion, which would involve dredging and increased coal ship traffic through Great Barrier Reef waters.

But the proposal was not included in a draft environmental impact statement (EIS) by the proponent, Queensland’s state development department.

It comes as a new International Energy Agency report on the world coal market cast fresh doubts on the future of Adani’s Carmichael mine, which would be Australia’s largest coal project.

The report noted that Carmichael led a group of proposed mines in north Queensland’s Galilee Basin which were all “highly dependent on availability of finance and adequate infrastructure”.

“Despite support from Queensland’s government, there is also public opposition to the (Galilee) projects, and, hence, the approval process is being challenged, with inevitable delays,” the IEA said.

“In conclusion, it is not likely that the above listed projects will be operational in 2020, if ever.”

The Environmental Defenders Office in Queensland has written to Hunt arguing that the bid for approval now appears to breach guidelines under the Environmental Protection and Biodiversity Conservation Act.

“Our client has concerns about material deficiencies in the EIS prepared … and provided as a basis for public consultation and your pending approval decision,” an EDO solicitor, Michael Berkman, said.

“Specifically, our client considers that the EIS fails to meet the requirements regarding the assessment of cumulative and consequential impacts of the project, in that it does not address the likely impacts of the tug harbour proposed to be constructed at the port of Abbot Point to meet increasing towage demand as coal throughput at the port increases.”

Berkman said documents obtained by NQCC under right to information laws showed that North Queensland Bulk Ports had planned the tug harbour for “a number of years … to meet increasing demand as a consequence of the anticipated increase in coal throughput, and that planning for the tug harbour appears ongoing”.

“Taken together, the information from the RTI documents indicates that the proposed tug harbour at the port of Abbot Point is likely to have potentially significant impacts on relevant matters of national environmental significance in the vicinity of the project, in addition to the project’s direct impacts that are assessed and reported in the EIS.”

Hunt has sought more information from the Queensland government on the matter after the EDO letter, the ABC reported. He faces a statutory deadline on Monday to rule on the Abbot Point project.

Previous plans for the Abbot Point expansion included ill-fated plans to dump dredge spoil in reef waters, then on the sensitive Calley Valley wetlands. The state development department now proposes to dump the spoil on a disused industrial site next to the port.

But the Palaszczuk government promised before its election this year not to allow dredging to go ahead until Adani had reached financial close on the project.

Last month Adani’s promoter, Gautam Adani, met the prime minister, Malcolm Turnbull, to urge him to press ahead with legislation restricting legal challenges by conservationists. Adani later complained to Indian media that the series of legal challenges thrown in the company’s way on its Carmichael mine in the Galilee basin and Abbot Point had deterred financial investors.

Berkman said it was “not entirely clear on the available information” if the state development department was aware of the proposed tug harbour development when submitting the EIS.

But he said EDO would investigate the matter and suggested Hunt “seek clarification on the proponent’s state of knowledge on this issue”.