The Australian Federal Police is investigating the leaking of information about Tuesday's raids on the Australian Workers' Union (AWU).

Key points: Leak of AWU raid details came from Michaelia Cash's office

Leak of AWU raid details came from Michaelia Cash's office AFP is investigating after Senator Cash requested police involvement

AFP is investigating after Senator Cash requested police involvement Attorney-General George Brandis said it would not be appropriate to discuss the issue further

Media crews were camped outside the powerful union's Melbourne and Sydney offices as AFP officers arrived to seize documents in connection with a Registered Organisations Commission (ROC) investigation.

Employment Minister Michaelia Cash's senior media adviser has resigned for tipping off the media that police were about to swarm on AWU offices in Sydney and Melbourne.

Senator Cash has been under pressure to resign because she repeatedly told Senate Estimates her office did not leak, but later corrected the record when her staff member confessed.

It is not yet clear who told the media adviser that the raids would happen.

Labor has focused its attention on a staff member inside the ROC who used to work for the Liberal Party.

Attorney-General shuts down questioning

Labor senators had hoped to ask more questions during an Estimates hearing in Canberra today, including to find the original source of the leak.

But as soon as the committee started, Attorney-General George Brandis said the AFP was now inquiring into the leak.

He said that meant it was not appropriate to discuss the issue during today's Senate Estimates hearing.

He tabled a heavily redacted letter Detective Superintendent Andrew Smith sent to Senator Cash's office last night and claimed was grounds for no further questions.

"As this matter is under investigation, it would not be appropriate to discuss the matter further," the letter read.

But Labor was quick to attack Senator Brandis for misinterpreting the letter.

Labor Senator Murray Watt said that meant the AFP would not be discussing it further, not that the Attorney-General or the Minister should not.

The line is similar to one in a statement the AFP sent to ABC News, which implied police would not answer any further questions.

Yesterday, Senator Cash told the committee she had written to the ROC which originally asked police to conduct the raids, suggesting it request an investigation into the leak.

The committee is sitting for an extra day today because of the controversy over the raids and the leak but Senator Cash is not appearing.

She has returned to her home state of Western Australia because she said she had longstanding commitments in Perth today.

Senator Brandis is appearing instead.

This morning, he told the committee that the AFP told Senator Cash's office last night that "as the matter is under investigation, it would not be appropriate to discuss the matter further".

He said that meant neither he, nor others would accept questions about the matter today "at the request of the Australian Federal Police".

Meanwhile, the union has launched a Federal Court bid to have the raids declared invalid.

But a hearing scheduled for today did not go ahead after the parties agreed to a timetable, with a full hearing likely in December.

It means any AWU documents seized by the AFP will not be handed over to the ROC before the December hearing.