'Possible criminal offence' in union raids

The Australian Federal Police believes a criminal offence may have been committed in relation to leaks about raids on union premises linked to Jobs Minister Michaelia Cash's office.

The AFP has referred evidence to the commonwealth prosecutor after investigating raids on Australian Workers' Union headquarters in Sydney and Melbourne last October.

"We do believe an offence may have been committed," AFP deputy commissioner Leanne Close told a Senate hearing on Friday.

The AFP is investigating the leak of information which led to media being tipped off and arriving before police raiding the offices.

Ms Close said dozens of people had been interviewed during the investigation, but refused to say if government ministers including Senator Cash had assisted police.

AFP officers raided the union offices last year as part of an investigation by the Registered Organisations Commission.

One of Senator Cash's media advisers quit after admitting to informing media about the raids.

Under questioning from Labor senator Murray Watt, Ms Close said it could jeopardise the investigation if she confirmed or denied if ministers had been interviewed.

She also declined to detail what crimes may have been committed and how many people could be implicated.

Liberal senator and committee chair Ian Macdonald said the AFP's refusal to rule out investigating Senator Cash proved nothing.

The minister has also claimed public interest immunity in answering questions over whether she has been investigated.

Labor's workplace relations spokesman Brendan O'Connor believes that explanation doesn't stack up.

"It is in the public interest to know if the AFP is investigating the role of the minister and her office in the leak and Senator Cash should come clean," Mr O'Connor said in a statement.

Earlier in the week, a spokeswoman for Senator Cash said there was no suggestion she was the subject of any investigation or referral of information to the prosecutor.

Senator Cash has repeatedly made clear she had nothing to do with alleged unauthorised disclosure of information.