Mystery still surrounds the control of one of Australia’s largest coal ports, which remains off the books of the Indian-based company that is listed as its owner with the Australian corporate regulator.

The Mumbai-listed Adani Ports is registered with the Australian Securities and Investments Commission as the ultimate holding company of the Adani Abbot Point coal terminal, known as T1, in north Queensland.

But this is at odds with Adani Ports’ latest annual report filed last week with the Indian stockmarket, which fails to list the $2bn port anywhere among its subsidiaries, with $2.2bn in associated debt also absent from its balance sheet.

It is the third year in a row that Adani Ports has disowned T1, which it bought via 99-year-lease in 2011. It remains the Adani group’s only cash-producing asset in Australia.

T1, which processes coal from other miners, is distinct from Adani’s proposed new terminal at Abbot Point, T0, which would process coal from its own controversial Carmichael mine in the Galilee basin.

The Carmichael project has been delayed by financing issues, a coal industry slump and legal challenges by traditional owners and conservation groups.

There is speculation that Adani will seek a federal government loan for a rail line linking its mine and port through the Northern Australia Infrastructure Facility, although the company has reportedly denied yet making an application.

Adani Ports announced the sale of its interest in T1 in 2013 for $235m to a private Adani family company in Singapore, which is in turn linked to a company in the Cayman Islands tax haven.

In every annual report since, Adani Ports has referred to Abbot Point only in a note recording the completion of the sale, saying: “The company has all the approvals except in respect of approval from one of the lenders who has given a specific line of credit.”

Tim Buckley, of the Institute for Energy Economics and Financial Analysis, said it was “bizarre that three years after Adani announced the transfer of ownership of one of Australia’s major ports from a reputable public listed company in India to a private family company, the transaction has still not been finalised”.

Buckley said there was a clear discrepancy between what Asic in Australia and Adani Ports’ shareholders in India were being told about the status of a major transaction.

This raised legitimate questions about corporate transparency at a time when Adani may be poised to seek Australian taxpayer subsidies for a key plank of its coal project, Buckley said.

Adani Mining’s Indian parent company, Adani Enterprises, is facing investigations by India’s Directorate of Revenue Intelligence (DRI) over allegations of profiteering on coal imports.

“The failure to clarify legal and economic ownership to Asic for more than three full years is testimony to Asic’s failure to regulate and the Adani group’s failure to comply with the spirit of Australian corporate law,” Buckley said.

The “opaque” nature of Adani’s corporate arrangements would complicate any efforts at due diligence by the Northern Australia Infrastructure Facility before funding the Galilee rail project, he said.

Guardian Australia has sought comment from Asic and Adani.

Rajesh Gupta, the financial controller of Australia-based Adani Mining, told the Queensland land court last year that the port “has not been sold yet” and was still owned by Adani Ports.

Gupta said approval of the sale by the Foreign Investment Review Board had expired.

Buckley said that if FIRB approval had lapsed, “why didn’t Adani Ports tell the Mumbai stock exchange and reverse the transaction?”

The new federal mining minister, Matthew Canavan, has described Adani’s Carmichael proposal as an “incredibly exciting project”.

The central Queensland senator said as minister he saw his primary job as facilitating the project to “make it easy as possible” while ticking the boxes on environmental, health and safety and community impacts.

Buckley said the latest financial snapshot of Adani Mining via records lodged with Asic showed it faced an uphill battle in trying to secure funding for a $10bn greenfield coal project.

Adani Mining, which currently has just 27 employees, had net debt in March of $1.4bn against shareholder funds of negative $227m, and combined losses over two years were $182m. Buckley said its Indian parent, Adani Enterprises, was also highly leveraged.

The Adani group has previously said it “strictly adheres to Australia’s robust environmental approvals, corporate governance and tax regulations”.

In April, an Adani group spokesman said it would cooperate with the DRI investigations but denied any wrongdoing and noted it had not been served with any “show-cause notice to date”.

An Adani spokesman told Guardian Australia in March last year it intended to proceed with the plan to change ownership of Abbot Point but that it remained “subject to financier approval”.