Opposition leader Bill Shorten is saying nothing about his union history, until he faces the Royal Commission later this year.

Allegations of dodgy deals are mounting against the Labor leader, who has been called to testify before the Royal Commission into Trade Union Governance and Corruption in late August or early September.

He is expected to front the hearing to answer questioned about deals brokered by the Australian Workers Union (AWU), which he led at both the Victorian and national level.

One of the most substantial claims centres on an AWU deal with a Melbourne construction company, which saw the company pay more than $38,000 to cover the union memberships of its employees, boosting the union’s numbers and political swing.

But some reports say the deal was approved by the industrial relations watchdog, and was all above board.

Mr Shorten will also be asked about his relationship with Victorian MP Cesar Melhem.

Mr Melhem stood down as Labor’s upper house whip in Victoria last week, after allegations he set up a union fund-raising company designed to aid electioneering and other political purposes.

The inquiry has also heard that the Victorian branch of the AWU artificially inflated its membership numbers through false accounting at a time when Mr Melhem was secretary.

In a 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.”

Mr Shorten will be the third consecutive Labor leader called to face a royal commission launched by the Abbott Government, when he fronts a royal commission that the Opposition considers another political witch hunt.