Opposition leader Bill Shorten has agreed to appear before the Royal Commission into Trade Union Governance and Corruption, after being asked late Thursday.

The royal commission contacted Mr Shorten's lawyers to ask if he would be able to testify in the next round of hearings in late August or early September.

The Labor leader is likely to be quizzed about deals brokered by the Australian Workers Union (AWU), which he led at both the Victorian and national level.

On Thursday, Mr Shorten said he would be happy to testify, following a report that the AWU had struck a deal with a Melbourne construction company, which involved the company paying more than $38,000 for union memberships for its employees.

"Ever since the royal commission was established by the Abbott Government, I've said I'd be happy to cooperate," Mr Shorten said in a statement.

"I welcome the opportunity to talk about my 21-year record standing up for workers."

A series of senior Coalition figures, including Prime Minister Tony Abbott, claimed Mr Shorten had questions to answer about the agreement.

He will also be asked about his relationship with Victorian MP Cesar Melham, who stood down as Labor's Upper House whip in Victoria this week, after allegations were aired about him during the inquiry.

In his statement, Mr Shorten said: "I will not be responding to matters that are being considered by the royal commission until I am able to appear.

"I have an absolute zero tolerance for corruption or criminality in the workplace — whether you are an employer, employee or union representative."

The Opposition has long called the royal commission a political witch hunt.

Mr Shorten will be the third consecutive Labor leader to front a royal commission initiated by the Abbott Government, but the only incumbent.

"Tony Abbott's abuse of taxpayers' money to serve a political agenda won't deter Labor from the fight for fairness," Mr Shorten said.

"I always put the interests of workers first as their representative."