Witness K appeared behind black screens in a blacked-out courtroom in the ACT Magistrates Court on Friday, where he faced sentence for a crime committed eight years ago.

In sentencing Witness K, Magistrate Glenn Theakston said it was not up to intelligence officials to unilaterally decide when to depart from their obligations of strict secrecy.

But he said it appeared the officer was not motivated by financial or personal gain, and instead a sense of justice.

As well as a three-month jail sentence, fully suspended, the magistrate ordered Witness K be of good behaviour for 12 months and pay security of $1000.

The former intelligence officer, now an elderly man aged above 70, had earlier pleaded guilty to conspiring with his lawyer Bernard Collaery to reveal secret information about the bugging operation.

Mr Collaery has pleaded not guilty.

In 2004, Australia and Timor Leste were negotiating a treaty over valuable oil and gas reserves. Timor Leste later challenged the treaty as invalid saying Australia had failed to act in good faith.

Witness K signed an affidavit to be filed with the court.

Mr Theakston said intelligence officers were subject to "strict and absolute" requirements for secrecy.

It was not up to the members to "unilaterally decide when to depart from that strict secrecy," Mr Theakston said.

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The magistrate did say that Witness K's actions were not for personal or financial gain, nor to compromise Australia's national security interests but for a sense of justice.

It was also not a "breach that was going to go hidden for long", given it was filed in a court dispute involving the Australian government.

The magistrate found Witness K was suffering from a number of mental health conditions at the time, and he "can't help but make a finding that his moral culpability is reduced".

He also needed to send a message to other officers that the "obligation to maintain that secrecy still prevailed".

Before the sentence, defence barrister Robert Richter QC had argued for Witness K to be spared a conviction.

Mr Richter said Witness K was elderly and suffered from a number of mental health conditions.

Not only should his moral culpability be reduced because he was suffering those conditions at the time, the delay in hearing his case had also exacerbated those conditions, he said.

He was motivated by a sense of justice for Timor Leste, the barrister said.

But Richard Maidment QC said Witness K needed to be made an example of to deter others.

The prosecutor told the court that the illegal disclosure was not a momentary lapse.

"This was a considered, continuing course of conduct," he said.

He said whether or not it an "act of altruism" or it was "appropriate" for Witness K to breach his obligations of secrecy was not a mitigating factor.

Mr Maidment said Witness K had originally sought out Mr Collaery, an advisor to Timor Leste, to act for him in a complaint against ASIS the "injustice" after being passed over for a promotion.

The prosecutor said there was no record of a complaint about Australian activities in Timor Leste with the Inspector General of Intelligence Services.

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