Letters and communications sent by Senator Michaelia Cash and her office illustrate a "keen political interest" in the Australian Workers Union before its officers were raided by police, a court has been told.
A high-profile Federal Court trial has begun to determine the legality of Australian Federal Police raids on the union's Melbourne and Sydney headquarters in October 2017.
The raids, which were part of an investigation by the Registered Organisation Commission, drew scrutiny because tipped-off TV crews and journalists arrived at the offices before police.
Senator Cash's former media adviser David De Garis admitted he leaked details of the raids to media.
The AWU claims the the investigation was illegal because it was politically motivated, allegedly instigated by Senator Cash to hurt the union and Opposition Leader Bill Shorten.
Senator Cash sent two letters of referral to the commission in relation to the AWU in the months before the investigation was launched, the trial was told.
"The underlying theme is there was a keen political interest on the part of the senator in the subject matter of the investigation," AWU's lawyer Herman Borenstein QC told the trial on Monday.
"It is demonstrated in the beginning by the letters and demonstrated again in October 2017 by communications ... and the media exploitation."
The commission's legal counsel Frank Parry QC argued the media leaks about the raids, which came three days after the investigation was launched, were not relevant in determining the trial.
He said the key issue was determining whether the investigation itself, launched by commission executive director Chris Enright, was deemed to be politically motivated.
"What goes on in the minister's office is a matter for the minister," Mr Parry said.
"Whatever views the minister might have had about political advantage ... take the matter nowhere as far as the Registered Organisation Commission is concerned."
Mr De Garis, who quit Senator Cash's office over the media leaks, began his testimony on Monday, detailing his work with journalists and ROC media representatives.
Senator Cash is expected to testify on Friday, with federal parliament sitting from Tuesday to Thursday.
"Michaelia Cash, after months of ducking and weaving, will herself give evidence," AWU's other lawyer Josh Bornstein said outside court.
"There will be nowhere to hide for Michaelia Cash. There will be no whiteboards in the Federal Court."
The raids were part of an investigation into a $100,000 donation from the union to activist group GetUp! in 2006.
Australian Associated Press
