Senior government frontbenchers are rallying around Employment Minister Michaelia Cash as pressure mounts on her to resign over her office's role in tipping off the media that police were about to swoop on the Australian Workers Union earlier this week.

An adviser to Senator Cash quit late on Wednesday after admitting to informing journalists of the imminent raid, which was mounted by the independent Australian Federal Police and Registered Organisation Commission. Labor has accused Senator Cash of misleading the Senate after she repeatedly denied her office was involved in any way.

Amid uproar about the leak of the information, Senator Cash wrote to the independent body suggesting the AFP investigate.

Labor frontbencher Tony Burke said the investigation into the AWU's donations to GetUp! a decade ago had begun as an attempt to smear Opposition Leader Bill Shorten and it had "blown up" in the Turnbull government's face.