Media leaks about raids on Australian Workers' Union headquarters will not result in criminal charges.

The Australian Federal Police referred evidence to the Commonwealth Director of Public Prosecutions last year after media outlets were tipped off in October 2017 about raids on the union's Sydney and Melbourne offices.

"The AFP can confirm the CDPP has advised they will not be proceeding with a prosecution as there are no reasonable prospects of conviction," the AFP said in a statement.

"The AFP considers this investigation finalised."

AAP

The raids drew scrutiny because TV crews arrived at the offices before police after a tip-off from then employment minister Michaelia Cash's media adviser David De Garis.

Mr De Garis later quit over the leaks, while Senator Cash has consistently denied any knowledge of the raids or tipping off journalists.

READ MORE Union raids from Michaelia Cash tip-offs referred to prosecutors

Police and officials from the Registered Organisations Commission were searching for documents relating to an AWU donation to activist group GetUp when Opposition Leader Bill Shorten headed up the union more than a decade ago.

The AFP told the Federal Court on Monday the CDPP would not pursue charges.

AAP

Labor's employment spokesman Brendan O'Connor said while he respected the independence of police and prosecutors, it was disappointing no charges were laid.

READ MORE Michaelia Cash to challenge subpoena in AWU raids case

"Senator Cash confirmed to the Senate that a staffer confessed to a crime," Mr O'Connor tweeted.

AAP

The fallout from the raids and subsequent questions in Senate estimates saw Senator Cash lash out - threatening to "sit here and name every young woman in Mr Shorten's office, over which rumours abound" while being questioned.

READ MORE Michaelia Cash uses whiteboard to shield herself from media after rumour threats

Labor Senator Penny Wong later demanded she retract the "disgraceful and sexist" remarks.

Senator Cash did withdraw the comments, but has not apologised.

- With AAP