ACTU head Sally McManus has called for Employment Minister Michaelia Cash to resign, as it emerged that a federal police investigation into raids on the Australian Workers Union will be referred to the Commonwealth prosecutor.

The Australian Federal Police has advised the union that it has been "liaising" with the Commonwealth Director of Public Prosecutions, and will formally refer at least one aspect of the investigation to it in coming weeks - meaning criminal charges could be laid.

"Michaelia Cash should have resigned a long time ago," Ms McManus told reporters on Monday afternoon.

ACTU secretary Sally McManus on Monday. Credit:AAP

"She's got no credibility on these issues; she is driven by her hatred of working people's organisations."