The Australian Federal Police has launched an investigation into the leaking of information about raids on two union offices, amid pressure on a federal minister to resign.

The decision comes as senior government figures continue to stand by the employment minister despite calls for her to step aside over her office's role in tipping off media about Tuesday's police raids on the Australian Workers' Union.

Attorney-General George Brandis, filling in for Employment Minister Michaelia Cash at a spillover Senate committee hearing in Canberra on Friday, said her office was informed the previous night that police were looking into the matter.

Citing correspondence from the AFP, Senator Brandis as the events were under investigation it would not be appropriate to discuss them further.

"Neither I nor any officer who may be called before the committee this morning will be accepting any questions in relation to that matter at the request of the Australian Federal Police," he told senators.

Senator Cash had a longstanding commitment in Perth on Friday and was unable to attend the hearing, the attorney said.

An AFP spokesman told AAP the agency was investigating the "alleged unauthorised disclosure of information" about the raids.

Senator Cash told the committee on Wednesday on five occasions neither she nor her office had anything to do the media tipoff.

She also personally told the prime minister she had not been the leaker.

But on Wednesday evening she announced her senior media adviser had resigned after revealing he had informed journalists of the raids on the AWU's Sydney and Melbourne offices.

Labor says she should resign or be sacked.

AFP officers executed the warrants, which had been sought by the Registered Organisations Commission to ensure documents weren't tampered with or destroyed.

The ROC told the estimates hearing on Wednesday night the AWU had refused to hand over all the documents it had requested, however on Thursday it retracted the statement.

The commission is investigating $200,000 in donations made by the AWU, including $100,000 given to activist group GetUp in the 2005/06 financial year while now-Labor leader Bill Shorten was the union's secretary.

The AWU has lodged a freedom of information request with Senator Cash's office to try and determine exactly when she learned about the raids.