Two more global banking giants have declared they will not participate in funding for the Abbot Point coal port expansion, adding fresh financial doubts for what are potentially Australia's biggest coal mines.

US-based Citi and Morgan Stanley have confirmed to opponents of the proposed Queensland port expansion near the Great Barrier Reef and the related coal mines in the Galilee Basin that they will avoid involvement in Abbot Port.

The Galilee Basin includes a prospective mine owned by India's Adani and GVK companies and also Clive Palmer, head of the Palmer United Party.

Abbot Point would be the main export point for mines that could ultimately export as much as 330 million tonnes of coal a year.

The mines have been dogged by uncertain finances, made worse by a coal price that has roughly halved over the past four years.

The need to dredge at least 3 million tonnes to expand Abbot Point has added to the environmental controversy, given the potential impact on the nearby Australia's World Heritage-listed reef park.

"Citi is not involved and does not plan to be involved in any financing for the Abbot Point expansion," Citigroup director, corporate sustainability, Valerie Smith said in a letter to be released on Tuesday by the Rainforest Action Network..

Morgan Stanley's executive director for risk management, Francesco Liberti, echoed the comments in a separate letter: "Morgan Stanley will not lend to or invest in the expansion of Abbot Point."

Two other US banks, JPMorgan Chase and Goldman Sachs, indicated they may also steer clear of the projects.

JPMorgan Chase said it would not bankroll natural resource projects within a World Heritage site without prior agreement between UNESCO and the government, while Goldman Sachs said it avoided financing any project that "would significantly convert or degrade a critical natural habitat".

The Queensland government has said the development of the Galilee Basin coal mines, including the rail and port facilities, would draw $28 billion in investment. The Abbot Point port expansion may cost at least $5.25 billion for the portion for just one of the mine developers, Adani Mining.

Adani, GVK response

Adani said Morgan Stanley's Australian unit confirmed to the miner that it had not examined "any of the Galilee Basin projects' fundamentals, nor made any assessment of the environmental or regulatory contexts", according to a spokesman.

"Morgan Stanley Australia are Adani's advisors on the potential partial sale of Adani's existing terminal (T1) at Abbot Point," the Australian-based spokesman said. "Any partial sale of Adani's current holdings at the port would - far from a withdrawal from the port - in fact be used to deliver the port's expansion."

Adani has also not sought funding for this project from Citibank, Goldman Sachs, or JPMorgan Chase, the spokesman said.

"The premise that institutions who have not been asked to provide funding, have no insight into our projects, and have not studied the details, have declined to or distanced themselves from involvement is rejected," he said.

The company also dismissed claims by green groups that the mines and ports would have damaging environmental impacts. "The approvals process for this expansion rank among the most rigorous and stringent ever applied in Australia, the USA or the rest of the world," the spokesman said.

GVK, the other major developer, said that any development of the existing Abbot Point port "will meet all obligations relating to responsible environmental and social management practices as outlined in the [UN's] Equator Principles".

The proposed expansion "will not impact the outstanding universal value of the Great Barrier Reef," the GVK spokesman said.

Lengthening list

The continued negative commentary from banks comes as Adani prepares to offer major tenders for some of the construction for its Carmichael coal mine and related operations within weeks.

Citi and Morgan Stanley's decisions on the port add to a lengthening list of banks, including Deutsche Bank and HSBC, that have declared they will steer clear of what would be the world's largest new coal developments. Concerns also include the projects' likely greenhouse gas emissions, weighing against the possibility of cheaper electricity for India and other nations.

Port developers agreed in September to seek an onshore site to dump the resulting dredge spoil from the planned expansion after widespread opposition to an off-shore site within the Great Barrier Reef Marine Park – despite approval from federal and state governments.

Thomas Jacquot, a credit analyst with rating agency Standard & Poor's, said the pullback of more banks would add to the difficulties already faced by the mine developers.

"The maths look challenging," Mr Jacquot said. "I'm ultimately very curious because the coal market is very depressed."

The ports, mines and railway need to be viewed as a whole, and doubts about the port's prospects will have knock-on effects, he said. "Nobody's going to lend to the port unless you know there's funding for the railway and the mines as well."

The projects' overall viability will hinge in part on whether Australian banks join the financing.

The Commonwealth Bank told Fairfax Media it had yet to be approached by Adani or any other party seeking finance for the Galilee Basin mines and related infrastructure.

"Should we be approached to consider financing, we would undertake all due diligence associated with environmental, social and governance [ESG] risks in light of our commitment to the [United Nations] Equator Principles III," a spokeswoman for the bank said. "We would not entertain financing projects that contravene our own ESG principle of not damaging a World Heritage site or other high-value conservation area."

A spokeswoman for NAB said the bank would apply a "robust due diligence process" to any major project.

Fairfax Media also sought comment from ANZ and Westpac.