Dexus suffered the setback on Friday after the NSW Supreme Court dismissed its attempt to retain at least part of its 27 per cent stake in Australia Pacific Airports Corporation (APAC), which owns Melbourne and Launceston Airports.
In 2024, the firm tried to find buyers for some of its shares in the corporation through an initiative titled Project Mercury.
The valuation of APAC was $13.8 billion, according to Dexus' own marketing material at the time.
During the sale process, the property investment firm provided confidential information to potential buyers and their advisers through a virtual data room.
In May 2025, APAC issued Dexus with a notice of breaching the shareholders deed, forcing it to sell its full stake in the enterprise.
While the firm challenged this notice in court, Justice David Hammerschlag found on Friday that it was valid.
"Dexus committed a material irremediable breach of the shareholders' deed, and the default notice is valid and effective," he wrote.
The confidential information was given, without permission, to at least 137 individuals and 40 organisations, the judge found.
Justice Hammerschlag found Dexus was required to act in good faith as a facet of APAC.
"Dexus irrevocably undermined the ongoing business relationship, trust and confidence between it, and the other shareholders," he wrote.
"APAC no longer trusts Dexus, and nor should it."
The firm leaked details to airlines including performance drivers, revenue, costs, identities of tenants, rent allocations and proposed initiatives.
While Dexus required potential buyers to sign its own confidentiality deed before accessing the virtual data room, APAC had no way to test the integrity of anyone who saw the private information, the judge said.
"The potential harm caused by Dexus' breach may not yet have come home to roost," he wrote.
The findings were made following an 11-day court hearing in April where over 28,000 pages of evidence were handed up to the court.
Dexus acquired its stake in APAC from AMP through a $275 million acquisition in 2023.
It went into a trading halt on Friday morning in anticipation of the court's judgment, when its shared were valued at $5.90.
In a statement filed to the Australian Securities Exchange, Dexus acknowleged the decision.
"(Dexus) are reviewing the judgment carefully to understand the court's findings and their implications and are considering grounds of appeal that may be available," the statement said.
"The potential impact of the judgment on Dexus is uncertain at this time".
The company is entertaining the possibility of asking the court to extend an injunction blocking its suspension from APAC until any appeal is filed.
An APAC spokesman welcomed the court's findings.
"(We) look forward to putting the issue behind us," he told AAP.
The shareholders in APAC that sought Dexus' ousting are an institutional investment manager owned by Australian industry superannuation funds, the custodian of Australia's sovereign wealth fund, the custodian of the NSW Treasury Corporation, and Utilities of Australia.
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