Ben Davies was the senator's chief of staff at the time of the October 2017 raids and on Monday confessed the "adverse effects on others" was front of mind when handling the matter.

Mr Davies received the "unexpected" tip from Mark Lee, who was then a media adviser on secondment to the Registered Organisations Commission and in the running for a job in Senator Cash's office.

It gave warning of raids by the Australian Federal Police on Sydney and Melbourne offices of the Australian Workers Union.

Following two referrals from Senator Cash, the commission was investigating the union over possible improper authorisation of two donations made when Mr Shorten was the AWU national secretary.

The raids - to prevent alleged evidence destruction - proved controversial when media arrived before police officers, after a tip from Senator Cash's then-media adviser David de Garis, who received the information from Mr Davies.

The union is taking action against the commission claiming the investigation was politically motivated and the raids were therefore unlawful.

Mr Davies said he knew the raids were likely to receive media coverage given their "political significance" and he gave Mr de Garis authority to notify journalists while trying to confirm the veracity of Mr Lee's tip.

He denied discussing benefits the raids or media coverage might have for Senator Cash.

"The thought was more on the adverse effects on others than the positive benefits to the minister," he said.

Mr Davies said he didn't discuss with Mr Lee where his information had come from, or if there was any expectation for what he'd do with it.

But he knew it was important given the rarity of raids of that type.

"There's obvious political implications as well, that the investigation relates to the period during which Mr Shorten was secretary of the AWU," Mr Davies said.

And there was an "obvious inference" that the union had covered up or attempted to destroy evidence that was unfavourable to Mr Shorten, he added.

Mr Davies conceded the tip had the ability to compromise the investigation but didn't think he had to stay quiet because the information "was in circulation".

However, he didn't pass it on to Senator Cash because she was in meetings and at that time the information was unsourced and unconfirmed.

Senator Cash told the hearing last week that the first she knew of the raids was seeing them unfold on television.

She also denied during heated questioning that she'd referred decade-old union donations to the commission watchdog as part of a political strategy to discredit or embarrass Mr Shorten.

Instead she said that after reading allegations in an August 2017 media report she felt it was incumbent upon her to ensure the watchdog was aware of the claims.

The hearings will continue later this month.

Australian Associated Press