A Labor frontbencher is demanding the resignation of Employment Minister Michaelia Cash after it was revealed she knew the head of the building watchdog breached the Fair Work Act almost a year ago.

Key points: Nigel Hadgkiss knew about material that suggested employers could stop union officials meeting in workplaces

Nigel Hadgkiss knew about material that suggested employers could stop union officials meeting in workplaces Employment Minister reveals she knew of the complain against Mr Hadgkiss in October last year

Employment Minister reveals she knew of the complain against Mr Hadgkiss in October last year Unions say revelations make "a complete mockery" of government's industrial relations agenda

Australian Building and Construction Commission boss Nigel Hadgkiss resigned today after admitting to breaching the act over right to entry rules.

In court it was revealed Mr Hadgkiss knew about material distributed by his organisation that suggested employers could make reasonable requests to stop union officials holding meetings in workplaces.

But under changes to the laws, the employer no longer had that right and unions could meet workers in meal rooms for talks if needed.

Unions reacted with fury to the admission.

And when pressed in Question Time, the Employment Minister revealed she had known about the matter for some time.

"I became aware of the behaviour in October 2016," Senator Cash said.

Labor senator Doug Cameron was furious.

"Not only should Mr Hadgkiss have resigned today, but the Minister should resign as well, she is responsible for him," he said.

"She should take responsibility for appointing someone who breached the law he was supposed to make sure was complied with."

Doug Cameron says Michaelia Cash "should take responsibility". ( ABC News )

Australian Council of Trade Unions president Ged Kearney was equally unimpressed.

"He was prepared to put up misinformation, it appears he wanted fact sheets to reflect laws he wanted to be in place rather than what was actually there," he said.

"It just makes a complete mockery of this Government's agenda to break down unions and to stop working people get justice at work."

Liberal senator and former employment minister Eric Abetz accused the union of seizing on a "slip-up" to distract from their own problems.

"That which Mr Hadgkiss is alleged to have done pales into absolute insignificance, it is literally in the parable, they are pointing out a little speck in Mr Hadgkiss's eye," he said.

Mr Hadgkiss has had a long career, heading up the Building Industry Taskforce, the office of the DPP in New South Wales, and the Construction Code Compliance Office in Victoria.

Senator Cash's office issued a statement to the ABC, but did not respond to specific questions about Mr Hadgkiss's actions.

"Minister Cash was first advised of the CFMEU's allegations against Mr Hadgkiss in October 2016," a spokesman said.

"Yesterday Mr Hadgkiss admitted to a contravention of the Fair Work Act. He has today submitted his resignation to the Government which the Government has accepted."