THE right of corporate whistleblowers to hide their identities has been boosted by a Federal Court appeal decision relating to a class action by Multiplex shareholders. Yesterday three judges unanimously agreed that the Australian Securities and Investments Commission could withhold documents sought by lawyers for the plaintiff shareholders.

The commission argued that allowing the documents to be used in the class action would disclose the identity of one or more employees of a company associated with the Wembley Stadium project in Britain who had informed ASIC that Multiplex had failed to disclose problems with the project.

Justices Peter Heerey, Michael Moore and Richard Tracey ruled there was "an undeniable public interest in the protection of the particular informer in question and the encouragement of potential future informers".

But they rejected the commission's request to suppress the reasons for granting public interest immunity, saying: "Not to publish our reasons would be contrary to the public policy behind our decision, which is at least as much to clarify the law for potential future informers as it is to protect the informer(s) in this case."

Last year the commission persuaded Justice Alan Goldberg to hear its arguments in closed court and to suppress most of his reasons for ordering the documents to be handed over, pending the appeal. Yesterday's judgment reveals that in his unpublished reasons, Justice Goldberg had said a critical issue was that the whistleblower's identity was known to his or her employer, who had funded legal representation for approaching ASIC.

"His Honour did not consider whether there was any real prospect of victimisation or adverse consequences being visited upon the informer(s) at the instigation of any other persons outside the [employing] company," the three judges said. "Rightly or wrongly, informers are often regarded with disfavour. Persons contemplating whistleblowing would realise that disclosure of their identity may cause them harm in ways they never find out - employment or promotions not offered, friendships undermined."

The judges also said the documents were not going to make a significant difference to the shareholders' case.

"The [class action] really boils down to an objective assessment of what in fact was happening at Wembley and whether or not that assessment should have been disclosed by Multiplex," they said.