Labor is demanding Employment Minister Michaelia Cash resign after she admitted one of her staffers tipped off the media about Tuesday's police raids on the Australian Workers' Union (AWU).

Senator Cash repeatedly told a Senate Estimates committee yesterday she and her office played no role in alerting the media to the raids.

But she returned to withdraw her comments in the evening after reports emerged her office had alerted journalists to the raids hours before they happened.

Senator Cash told the committee her senior media adviser had confessed to tipping off journalists.

Labor has accused her of misleading Parliament and said she had no option but to quit the ministry.

But Senator Cash said her media adviser acted without her knowledge or authorisation, and has now resigned.

"I was not aware of it at the time and was not aware of it earlier today in [Senate] Estimates," she said.

"This took place without my knowledge and was not authorised by me. I was not notified of the raids until I watched them unfold on television.

"My staff member has now resigned."

Sorry, this video has expired Cash did nothing wrong says Pyne

Senator Cash said her media adviser learned about the raids from a "media source" — but said she did not know if he was referring to a journalist, or to a media adviser who worked with another organisation.

Police raided the union as part of an investigation into donations made more than a decade ago to the lobby group GetUp, and to Labor candidates.

Bill Shorten headed the AWU at the time and the ALP has accused the Government of orchestrating the raids to smear the Opposition Leader.

Labor senators piled pressure on Senator Cash during the estimates hearing, pointing out she had repeatedly assured the committee her office only learnt about the raids shortly before they happened.

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Sorry, this video has expired Burke says Cash's denial 'defies credibility'

But Senator Cash said she did not knowingly mislead Parliament.

"I was unaware of the course of conduct that was followed by my staff member, so my evidence stands," Senator Cash said.

"Everything I've stated is based on the knowledge that I knew at the time.

"Are you saying that means you have no responsibility, is that what you're saying?" Labor Senator Doug Cameron asked.

"I can only provide answers based on my knowledge at the time. At the earliest opportunity that I could address this committee I have," Senator Cash replied.

But the Opposition has scoffed at her explanation and is demanding her resignation.

Labor frontbencher Tony Burke said it "defied credulity" that Senator Cash's staffer said nothing while she repeatedly gave misleading evidence.

"When Labor first asked Senator Cash whether or not her staff, her office, had been involved in making sure that the media turn up to the raid before the police did, she said that she was offended by the suggestion. She ridiculed Labor for asking it," Mr Burke told the Lower House.