Employment Minister Michaelia Cash is in Labor's crosshairs after she admitted one of her staffers gave advance notice to journalists about a police raid on the Australian Workers' Union (AWU).

We've gone back to the beginning to show you how we got here and why it's significant.

First, we have to go back to the creation of the Registered Organisations Commission (ROC)

This is the regulator responsible for scrutinising how unions are run.

It's only six months old. The legislation to create it was passed last November.

At that time, Senator Cash denied it was about "union busting", instead arguing that the ROC was necessary for "increasing transparency and accountability".

Labor hadn't supported the legislation because it wanted a number of amendments, including making the Australian Securities and Investments Commission (ASIC) the regulator of union officials instead of the ROC.

Fast forward to the raids on AWU offices in Melbourne and Sydney on Tuesday

Australian Federal Police (AFP) officers carried these out on behalf of the ROC, who said the warrant was issued that morning.

The investigation is about whether donations to GetUp and federal Labor campaigns were authorised under union rules.

The donations were made during the financial year ending 2006.

Opposition Leader Bill Shorten was the head of the AWU at the time.

Media had been tipped off that the raids were happening and were at the scene to cover them.

Mr Shorten called the raids a "grubby attempt" by Prime Minister Malcolm Turnbull to smear his reputation.

Senator Cash's troubles started during Senate Estimates yesterday, when she said her office had nothing to do with the leak

Senator Cash was grilled by Labor senator Doug Cameron on this. But she told him:

"I can assure you that I found out about the raids as they unfolded on the television. I can also assure you that my office did not find out about the raids until after they were being conducted."

She also said she was "offended on behalf of my staff".

"They are very serious allegations and I refute them," she said.

But hours after Senator Cash's first assurance to the contrary, Buzzfeed reported that journalists had been tipped off by her office

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Shortly after that, Senator Cash confirmed that one of her staffers had in fact given information to journalists

Senator Cash said her senior media adviser told her this during a dinner break.

"This took place without my knowledge and was not authorised by me," she said.

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

She said she was told by the staffer that they had got the information from a "media source".

Senator Cash said she had not been aware of any of this earlier in the day. The staffer involved resigned.

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This photo of Senator Cash during Senate Estimates last night said it all:

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This morning, Labor called for Senator Cash's resignation

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

Manager of Opposition Business Tony Burke told ABC News Breakfast:

"The Prime Minister has to show leadership here and fully understand and appreciate the consequences of the Minister and her office and seek her resignation today."

But Coalition ministers came to her defence.

"Michaelia Cash told the Senate the truth, and as soon as she found out that she'd been misled, she corrected the record," Defence Industry Minister Christopher said.

Social Services Minister Christian Porter said of Senator Cash, "she is absolutely to be believed".

Senator Cash said she wasn't considering stepping down

Meanwhile, she told Senate Estimates that she had asked the ROC to consider asking the AFP to investigate how information about the raids was leaked.