It is an intriguing corporate web that spreads from North Queensland, across Asia to the Caribbean.

Giant Indian conglomerate Adani, which plans to build one of the world's largest coal mines in Queensland's Galilee Basin, has set up a complex network of companies and trusts in Australia which are owned in one of the world's major tax havens, the Cayman Islands.

The Adani Group is also attempting to shift ownership of the existing Abbot Point coal port — which it bought for $1.8 billion — to a Singaporean company ultimately owned in the Cayman Islands.

An exhaustive search of company filings and documents across the globe has cast light on this opaque structure of ownership and control.

It has alarmed environmental activists and legal experts, who fear it could make it harder to gain compensation from Adani in the event of an environmental disaster from Adani's planned mine and port expansion on the edge of the Great Barrier Reef.

"I've been a businessman for most of my life, as well as an environmental activist, and the risks are great," said Geoff Cousins, former Optus CEO and chairman of the George Paterson advertising agency, now a board member of the Australian Conservation Foundation.

"With these kinds of approvals of big mining operations or port operations, you always get a set of conditions that the Government puts on.

"But those conditions aren't worth anything if, when something goes wrong, you try to find the company responsible and either it has no money or if it has money it's in a tax haven and you can't reach it."

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It is a view echoed by David Chaikin, a professor of business law at the University of Sydney.

"The advantage of having the money in tax havens is that you are able to conceal the source of money, the use of money, and also to minimise tax," he said.

Coal infrastructure owned through opaque structures

As well as building Australia's biggest coal mine in north Queensland, Adani is planning a huge expansion of the existing coal terminal at Abbot Point, near Bowen, to ship coal across the Great Barrier Reef to India — turning it into one of the world's biggest coal ports.

It also wants to build a new railway linking the mine, about 400 kilometres inland, to the port.

All the planned developments are based on corporate structures involving tax havens.

Control of the railway — which the Federal Government is preparing to support with a $1 billion publicly subsidised loan — ultimately resides in the Cayman Islands, one of the world's most notorious and secretive tax havens.

Adani has created four companies and two trusts in Australia for the rail project.

The parent company for all these entities is Carmichael Rail and Port Singapore Holdings Pte Ltd, a company registered in Singapore where the corporate tax rate is 15 per cent.

This Singapore parent company is in turn owned by Atulya Resources Limited, a private company controlled by the Adani family and based in the Cayman Islands.

The port expansion has a similar structure: five companies and two trusts in Australia, ultimately controlled by Atulya Resources in the Cayman Islands.

The vast expansion of the coal port planned by Adani has sparked enormous controversy.

It will involve dredging 1.1 million tonnes of spoil from the ocean near the Great Barrier Reef Marine Park and poses a potential danger to the environmentally-sensitive reef, which is listed on the World Heritage Register.

Critics say the company structure set up by Adani raises serious concerns about the value of strict environmental approvals placed on the project.

Abbot Point ownership in limbo

Ownership of the existing Abbot Point Coal Terminal is in limbo.

Adani bought a 99-year lease over the coal port in 2011 for $1.8 billion through a company listed on the Bombay Stock Exchange, Adani Ports and Special Economic Zone Ltd (AZPEZ).

That company said it "sold" the port three years ago to a Singaporean-based Adani family company "subject to regulatory and lenders approvals".

But the sale has not been completed, because of objections by the State Bank of India, which lent Adani $US800 million ($1.1 billion) for the port purchase.

In its latest filings with the Australian corporate watchdog, Adani still lists the port as being owned by the Bombay-listed company.

But ASPEZ's 2016 annual report said it had "recorded the divestment" of the port to Abbot Point Port Holdings Pte Ltd, Singapore: an entity which lists as its sole director Vinod Shantilal Adani, the brother of Guatam Adani, head of the Adani Group, and which is ultimately owned by Atulya Resources in the Cayman Islands.

Transferring ownership of the critical port infrastructure to a Caymans Islands' company "means it will be unregulated, unaccountable," Tim Buckley, director of the Institute for Energy Economics and Financial Analytics told the ABC.

"It will be non-transparent to the Australian Government as to what is going on, who owns it, who are the directors. To me it is a matter of national security."

Companies and trusts created by Adani for the proposed Carmichael mine are ultimately owned by Adani Enterprises, a publicly-listed company in India, but the control flows via a company registered in the tax haven of Mauritius, Adani Global Ltd.

A $5 billion fund the Federal Government set up for the development of northern Australia, the Northern Australia Infrastructure Facility or NAIF, is considering a request from Adani for a $1 billion subsidised loan for its rail development.

The NAIF refused to disclose which Adani entity had applied for the finance when approached by the ABC.