Senator Michaelia Cash denies having any knowledge of controversial police raids on union offices until she saw them in the news.

Two staff in her office have been named as the source of tip-offs to the media about the October 2017 federal police raids on Australian Workers' Union offices in Melbourne and Sydney as part of an investigation by the Registered Organisations Commission.

She told the court in Melbourne on Friday the first she knew of the raids was seeing them unfold on television.

And she believes she only became aware during Senate estimates the following day that media, who arrived at the offices before Australian Federal Police officers, were given a heads-up.

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The cabinet minister is giving evidence to a Federal Court trial to determine the lawfulness of the raids, which the union claims were a political play to harm the profile of Labor leader and former union secretary Bill Shorten.

Senator Cash's former media adviser David de Garis and ex-chief of staff Ben Davies have been implicated as sources of the tips.

The commission was investigating whether the union broke its rules by failing to get authorisation for two donations in 2005 and 2006, to activist organisation GetUp! and Mr Shorten's election campaign.

Senator Cash said she first heard about those allegations through a media article in 2017, and despite knowing she couldn't direct the independent commission to investigate, she felt it was incumbent upon her to make sure the watchdog was aware of potential wrongdoing.

She wrote two letters to the commission but didn't receive a response.

AAP

Senator Cash said she couldn't recall ever making other suggestions of referrals to the commission.

She repeatedly denied having any interest in the naming of Mr Shorten in allegations, beyond the fact he was the union's secretary at the relevant time.

She said her interest in the matter was in discharging her duty as employment minister.

"I am a politician and it would have been of potential interest to me as a politician," she conceded when pressed about her general political interests.

Senator Cash revealed in the month before he raids she had authorised a job offer for Mark Lee, a media adviser with the Fair Work Ombudsman, to become her senior media adviser.

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Mr Davies on Thursday named Mr Lee as his source of details about the union raids.

Then-acting media adviser, Mr de Garis, said information was passed to him by Mr Davies, and together he collided with Michael Tetlow - the media adviser to then justice minister Michael Keenan - to notify media.

Mr Lee ultimately didn't take up the role.

Senator Cash said Mr Davies told her Mr Lee "didn't want the role" a week after the raids "because his name had been raised in Senate estimates he didn't want to come work in a minister's office".

The trial continues.