Ernst Willheim asked the ACT Supreme Court on Thursday to allow him to make an open justice case during the trial of Bernard Collaery, the former lawyer to spy-turned-whistleblower Witness K.

The pair are facing conspiracy charges for revealing Australian spies bugged the East Timorese government's cabinet room.

Mr Willheim, advocating for open justice, made a bid as an "amicus curiae".

He was concerned Mr Collaery's case would be heard behind closed doors over national security fears.

An amicus is not party to a case but can offer advice or expertise to the court.

Justice David Mossop shut down the attempt, with Mr Collaery's lawyers saying they didn't oppose the bid but said it was up to the court.

Mr Willheim, a law lecturer, said an open trial was "fundamental to the public confidence of the administration of justice".

"It is especially relevant in a case such as this," Mr Willheim said.

But Justice Mossop said he was already "acutely conscious" of having matters dealt with in public and Mr Collaery would likely advocate for an open trial.

The judge shut down Mr Willheim's application and said he would deliver his reasons why at a later time.

But he told the court the parties involved wouldn't have to be present, to prevent them "expending" more time on the matter.

Lawyers representing the attorney-general opposed the bid, despite not opposing or supporting a similar attempt to intervene by Mr Willheim in the separate case of Witness K.

Witness K is expected to plead guilty to the conspiracy charges he faces while Mr Collaery has said he would fight his charges.

Hearings around how to handle top secret national security information in Mr Collaery's case are ongoing, with the matter set to return to the court next Thursday.

Witness K's case will return to the ACT Magistrates Court in November.

Australian Associated Press