The new head of Australia's peak union body is standing by her controversial comments on breaking the law.

Key points: Coalition frontbenchers lined up to criticise new ACTU secretary Sally McManus

Coalition frontbenchers lined up to criticise new ACTU secretary Sally McManus Opposition Leader Bill Shorten backed Ms McManus, distanced himself from comments

Opposition Leader Bill Shorten backed Ms McManus, distanced himself from comments Australian Manufacturing Workers Union supported Ms McManus on social media

Australian Council of Trade Unions (ACTU) secretary Sally McManus stoked debate when she said that she had no problem with unions breaking "unjust" laws.

"I believe in the rule of law when the law is fair and the law is right," Ms McManus told 7.30, in her first major television interview after taking the position.

"But when it's unjust I don't think there's a problem with breaking it."

That provoked a furious response from Coalition frontbenchers who have lined up to criticise Ms McManus.

Defence Industry Minister Christopher Pyne accused her of reciting "anarcho-Marxist claptrap" and said there would be "chaos" if Australians failed to follow the law.

Today Ms McManus issued a new statement clarifying her comments — but stood by her central argument that unions are justified when they break the law in order to improve conditions for workers.

"Australia has been built by working people who have had the courage to stand up to unfair and unjust rules and demand something better," she said.

And she argued the union movement only won fundamental workplace rights by defying the law and taking industrial action.

"Every single Australian benefits from superannuation, Medicare, the weekend and minimum wages — these were all won by our parents, grandparents and great-grandparents taking non-violent so-called illegal industrial action," Ms McManus said.

"Working people only take these measures when the issue is one of justice, like ensuring workers' safety on worksites, a fair day's pay for a fair day's work or to uphold and improve rights for working people."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 8 minutes 38 seconds 8 m New ACTU boss says it's ok for workers to break 'unjust laws' ( Leigh Sales )

Shorten backs union leader but distances himself from comments

The Opposition Leader Bill Shorten has distanced himself from Ms McManus' comments, saying he did not agree that it was justifiable to break the law.

"There are unfair laws in workplaces, I don't disagree with that summary, I don't support the cuts to penalty rates. But the way you best handle unfair laws is you change the law, and the best way to change an unfair law is to change the Turnbull Government," Mr Shorten said.

But he has continued to back the new union leader, saying she would do a "good job" in the position.

Employment Minister Michaelia Cash said Mr Shorten's comments were a "significant admission" that he was planning to roll back industrial relations laws put in place by the last Labor government.

"Mr Shorten needs to make clear which of Labor's own laws are the "bad laws" that he now says need to be changed, and why he is allowing law-breaking unions to determine his workplace relations agenda," Senator Cash said.

"Bill Shorten is yet again proving himself to be the weakest Labor leader in generations."

Some Federal Labor MPs have been scornful about Ms McManus's comments.

"It's laughable for her to suggest that she can be the arbiter [of this question]," one ALP backbencher said.

"It was incredibly clumsy at best and deliberately inflammatory at worst. [She] must have expected the question, and she chose to grandstand."

But other Labor MPs have defended the new ACTU secretary, arguing that unions have often taken illegal industrial action and then had their actions vindicated.

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Major unions have also rushed to defend Ms McManus.

The Australian Manufacturing Workers Union took to social media to list famous union campaigns which breached the law.

"We apologise sincerely on behalf of our comrades across the country and throughout time for breaking the law so gotdam much" the union tweeted.

"Let us know where we can return your superannuation, eight hour day, job security, SUBURBS OF SYDNEY, and equal pay."