Federal Employment Minister Michaelia Cash has suggested the Australian Workers' Union raids leak should be investigated by the Australian Federal Police.

Key points: Cash told Senate Estimates last night her staffer gave media notice of AWU raids

Cash told Senate Estimates last night her staffer gave media notice of AWU raids Today, Cash wrote to Registered Organisations Commission, asking them to consider referring leak to AFP

Today, Cash wrote to Registered Organisations Commission, asking them to consider referring leak to AFP Labor is calling for Cash's resignation

Last night, a staffer from Senator Cash's office resigned after admitting he had given the media advanced notice the AFP was raiding the Sydney and Melbourne offices of the AWU.

Labor is demanding the Prime Minister sack Senator Cash because she initially told the Senate Estimates Committee her staff were not responsible for the leak.

Her senior Cabinet colleagues have rejected any need for her to be sacked.

Senator Cash told the committee this morning she has not considered resigning over the matter.

This morning she wrote to the Registered Organisations Commission, which is running the investigation into the AWU.

She asked the commission to consider referring the leak to the AFP.

Last night she revealed to the committee that her senior media adviser had admitted he spoke to journalists ahead of the raids.

"Without my knowledge one staff member in my office, in the course of discussions with journalists, indicated that he had received information that a raid may take place," she said.

Sorry, this video has expired Michaelia Cash says leak happened without her knowledge

The raids were requested by the Registered Organisations Commission, which is examining whether the AWU broke donation rules more than a decade ago, when Opposition Leader Bill Shorten was secretary.

The donations were made to activist group GetUp and three federal Labor campaigns, including Mr Shorten's.

The AWU and Labor claim the investigation is an attempt by the Federal Government to damage the reputation of Mr Shorten and so the Opposition is doing everything it can to hurt the Coalition.

After she made the admission, Ms Cash faced intense pressure from ALP Senators in a fiery Senate Estimates hearing.

Labor Senator Murray Watt questioned why it took several hours for Senator Cash to explain.

"Over the course of today since you have begun making these denials, there has been a morning tea break, a lunch break, an afternoon break, you're sitting there with an iPad, a laptop and a mobile phone, and not at any point did your staff bring this to your attention until the dinner break," he said.

Ms Cash insisted she had not knowingly misled the Senate.

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

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

But Labor frontbencher Tony Burke rounded on the frontbencher in Parliament, demanding her resignation.

"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.

"There might be a member of Senator Cash's staff who's just resigned, but the wrong person has resigned."

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

Commission denies leak came from its office

The ALP also grilled the ROC about whether it tipped off Senator Cash's office about the raids.

The Commission's executive director Chris Enright said he only informed her just before the raids began, and made it clear he was deeply unhappy about the media presence.

"There is no person in this country who is more disappointed than me when I heard that [the media were there]," he told the committee.

Mr Enright said he would speak with his staff to ensure they had not leaked the information, but brushed off suggestions the information came from his office.

"I would be very confident, more than confident if I could put it that way, more than confident that it could not have come from my office," he said.

The ROC said it launched the raids because it had information suggesting documents were being tampered with or destroyed.

Labor senators demanded to know where those details came from.

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

The Registered Organisations commissioner Mark Bielecki said it came from a "caller" who got in touch with one of his staff members.

But he would not provide any details about when the call was made or who made it, arguing that could compromise the investigation.

The union watchdog also accused the AWU of withholding documents it needs to conduct its investigation into the donations.

The Opposition pressed the ROC on why it did not simply ask the union to hand over the documents it needed.

Labor Senator Kimberley Kitching said the union had made it clear it would have been happy to give the ROC everything it wanted.

But Mr Enright told Senator Kitching the union had refused to hand over some papers.

"The AWU through its lawyers expressly declined my request to provide the documents that I requested," he said.

"Again I ask you — why didn't you use a notice to produce, why didn't you use that mechanism?" Senator Kitching said.

But Mr Enright said he could not provide any further information because the matter was before the courts.