The ACT Court of Appeal ruled in October that the trial of Mr Collaery, a Canberra lawyer charged over his role in the exposure of an Australian espionage operation in impoverished East Timor, should be conducted largely in open court.

The decision overturned an earlier ACT Supreme Court ruling of Justice David Mossop, who had, in June 2020, granted then-attorney general Christian Porter's application for significant parts of the trial to take place behind closed doors.

Three Court of Appeal judges accepted that the public disclosure of some evidence would, as Mr Porter had argued, involve a risk of prejudice to Australia's national security.

But, in a strongly worded judgment summary, Chief Justice Helen Murrell, Justice John Burns and Justice Michael Wigney said they doubted whether a significant risk would actually materialise.

Secrecy, however, posed a real risk of damage to public confidence in the administration of justice, they said, while open justice deterred "political prosecutions".

Beyond the one-page judgment summary published on October 6, the Court of Appeal's reasons are unknown.

The full judgment was going to be published on November 5, after Chief Justice Murrell had heard from lawyers involved in the case and made what she considered the appropriate redactions to a version that would be released online.

But Tim Begbie QC, a barrister for current federal Attorney-General Michaelia Cash, told the Court of Appeal that day that a challenge was being considered and his client may ask the High Court to apply greater secrecy to the judgment.

Chief Justice Murrell described this as "rather unusual" and encouraged the Attorney-General's department to "bring all its resources to bear" in order to make a swift decision.

She was unable to publish the judgment as a result of the mooted appeal.

Senator Cash had 28 days to apply to the High Court for special leave to appeal against the decision on the redactions.

On Friday, the last day available to do this, her lawyers filed such an application.

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The High Court will now need to decide whether or not to hear the appeal.

Kieran Pender, a senior lawyer at the Human Rights Law Centre, described Friday's development as "yet another perverse turn" in Mr Collaery's case.

"The Attorney-General's attempt to have a decision that said no to secrecy itself be kept secret is not only absurd but undemocratic," he said.

"Rather than seeking to shroud this prosecution in secrecy at every turn, the Morrison government should be reforming the law - like it promised to do 12 months ago - so that whistleblowers who speak up are protected, not punished."

Justice Mossop, meanwhile, is due to hand down another a key decision in Mr Collaery's case this coming Tuesday afternoon.

It relates to whether Senator Cash will be able to update so-called "court-only evidence", material so secret not even Mr Collaery or his lawyers have been allowed to see it, ahead of the 77-year-old's eventual trial.

Mr Collaery's former client, an ex-Australian spy known only as Witness K, was sentenced earlier this year to a suspended jail term over his role in the 2004 bugging of East Timor's government becoming known to the public.

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