Small Business Minister Michaelia Cash faces a grilling in the witness box on Friday over the plot to tip off the media over union raids designed to embarrass Bill Shorten.

It’s a showdown the Australian Workers Union has spent years pursuing since the 2017 raids, obtaining a subpoena to force her to appear.

Union lawyers predicted this week there will be no “whiteboards” for the cabinet minister to hide behind, as she was once accused of doing at Senate estimates.

Senator Cash repeatedly evaded questions in Parliament on Thursday about revelations in court over the role of her former chief of staff Ben Davies and former media adviser David de Garis in leaking the raids, vowing to respect the court process.

The political drama played out in two parallel forums on Thursday – in Parliament, where Ms Cash refused to respond to revelations in the court on the grounds she is a witness in the civil trial, and in court, where her former staffers were forced to reveal the list of Turnbull government staffers involved.

“As I am yet to give evidence to the case I am subject to restrictions in relation to what I can be told about the evidence that is being given by other witnesses,’’ Senator Cash told the Senate.

On Thursday, the court heard that the source of the original leak to her chief of staff Ben Davies in 2017 was a media adviser called Mark Lee, who was working for the Registered Organisations Commission and had just been offered a job in Senator Cash’s office.

“It’s been revealed Mark Lee, as acting media adviser for the ROC, informed Senator Cash’s office about the AWU raids. More proof this government set up and politicised an agency to do its dirty work and attack its perceived political enemies. The government is up to its neck in this scandal,” opposition workplace relations spokesman Brendan O’Connor said.

The revelation prompted the opposition to ask Senator Cash a string of questions in Parliament, which she declined to answer pending her evidence to the court.

“The statement I made to the Senate Estimates Committee was based on the advice given to me. The AWU’s legal action is in the course of hearing before the Federal Court of Australia this week,’’ Senator Cash said.

“Although Senator Wong and other Labor Senators have provided commentary on the case during question time, I have no time to verify if what Senator Wong has said is correct.”

Senator Cash has previously told the Senate in 2017 that the original leak came from a “media source”.

”I have been advised that without my knowledge (Mr De Garis) indicated he received information a raid may take place. I’m advised this came ‘from a media source’,’’ she said.

Mr De Garis has told the court he also rang another media adviser in then justice minister Michael Keenan’s office to divvy up responsibility for telling television journalists and print journalists to maximise media coverage of the raids.

This staffer, called Michael Tetlow, called television stations and Mr De Garis tipped off The Australian, The Daily Telegraph and Fairfax.

Mr De Garis’s evidence is in direct contradiction with Mr Keenan’s assertion to Parliament that none of his staff were involved in the leaks.