Senator Michaelia Cash's chief of staff Ben Davies was the source of a controversial leak to media about imminent police raids at Australian Workers' Union offices.
David de Garis, a former media adviser to the senator, was ordered to name his source by Federal Court Justice Mordecai Bromberg on Tuesday.
Mr de Garis resigned from the position after passing information to journalists who arrived at union offices in Melbourne and Sydney before Australian Federal Police in October 2017.
He admitted on Tuesday that he didn't understand the consequences of his actions at the time, but had wanted to get media coverage of the raid.
Mr de Garis claimed the raids were significant because the investigation of the union was one of the first for the new Registered Organisations Commission, and had been viewed as significant for the senator and staff for the same reason.
His naming of Ben Davies - due to give evidence later this week - came as it was revealed the former staffer was taking his own legal action against the union in a defamation suit.
Mr Davies lodged his action in the Federal Court in NSW on Friday, naming the union's national secretary Daniel Walton and Buzzfeed journalist Alice Workman as respondents.
An initial hearing is due to take place in March.
Mr Davies no longer works for Senator Cash.
The ongoing case by the union against the commission stalled on Tuesday as AWU lawyers sought to expand on their original submissions.
The union is arguing AFP raids as part of the commission's investigation were unlawful, alleging they were politically motivated and instigated by Senator Cash in a bid to hurt the union and Opposition Leader Bill Shorten.
AWU legal counsel Herman Borenstein QC said that evidence still to come from Mr Davies and commission executive director Chris Enright could provide a basis for an allegation that the latter acted for a purpose that Senator Cash wanted to achieve.
He said Senator Cash had no statutory power to direct the commission, but two cases of her doing so and the commission acting on her directions, existed.
Mr Borenstein said it raised serious questions about the commission and suggested an independent regulator might be expected to respond that "we don't take direction from you".
The delay means Mr de Garis is set to return for a third day of evidence, when Mr Borenstein hopes to press further on the conversation in which Mr Davies revealed detail of the raids.
Mr de Garis was able to recall that Mr Davies told him "around 4pm there would be the execution of raids" but not what his response had been or if he was told what, if anything, he should do with that information.
The conversation lasted only a few minutes and happened in Mr Davies' office on the day of the raids, he added.
Australian Associated Press
