Another former staff member of Senator Michaelia Cash is being pressured to reveal what he knew of raids on Australian Workers' Union offices, while the senior Liberal remained tight-lipped in parliament.

A Federal Court trial continued on Thursday to hear evidence about the lawfulness of Australian Federal Police raids on AWU headquarters in Melbourne and Sydney in October 2017.

The raids were part of investigations into the legality of $100,000 in donations the AWU made to left-wing lobby group GetUp! in 2006 when the union was led by now-Opposition Leader Bill Shorten, who also sat on the lobby group's board.

Senator Cash's then-chief-of-staff, Ben Davies, was asked who told him the raids would occur, but refused to answer.

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"I respectfully decline to answer on the basis of self-incrimination," Mr Davies told the court.

"I do not wish to give evidence willingly."

His lawyer Richard Dalton SC argued that even if the court offered protection against prosecution, that was insufficient.

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There was a "real risk of Mr Davies getting prosecuted" under parliamentary contempt laws, Mr Dalton said.

Senator Cash was similarly reluctant to answer questions on the matter in the upper house.

She confirmed her court appearance in Melbourne on Friday.

"As I am yet to give evidence in the case, I am subject to restrictions about what I can be told about the evidence that is being given by other witnesses," Senator Cash replied when asked about revelations already made by her former media advisor David de Garis.

He told the court earlier this week he learnt about the raids from Mr Davies, before colluding with then-justice minister Michael Keenan's staffer Michael Tetlow to tip off media outlets so cameras and reporters arrived before police on the day of the raids.

Mr de Garis also admitted the media attention was at least in-part designed to make Mr Shorten look bad.

In the lower house, Mr Keenan was more willing to talk, seizing the opportunity to land a blow on Mr Shorten.

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"When he was running the AWU, and this is what the police were investigating, there is allegations that union members' money was used in a cavalier fashion," he said.

"(Money was) given out to GetUp!, given out to his own campaign fund, and he has never once taken the opportunity to actually say why he won't allow that evidence to be examined by the police."

In the trial, the AWU is arguing in its case against the Registered Organisations Commission and AFP that the raids were unlawful.

Federal court judge Mordecai Bromberg will on Friday make a decision on whether Mr Davies will be compelled to give evidence.