The royal commission into trade unions is investigating whether the Victorian branch of the Australian Workers Union (AWU) artificially inflated its membership numbers by charging cleaning business Cleanevent up to $25,000 a year.

In his opening statement, counsel assisting Jeremy Stoljar SC said: "Instead of securing better wages or penalty rates for members, some officials may have preferred to obtain payments which strengthened the union balance sheet and which falsely inflated membership numbers."

The commission heard that in 2010 Cleanevent began working with the union to extend an Enterprise Bargaining Agreement that saw workers paid below award wages, banned industrial action and saved the company millions of dollars.

Mr Stoljar read an email dated June 2012, in which Cleanevent's general manager of operations said: "The $25,000 was part of that negotiation ... the $25,000 is an annual cost."

"The implication to the business by not having the EBA and employing labour through the modern award is circa $2 million per annum," the email said.

Mr Stoljar told the commission that cleaners' names were supplied to the union without their knowledge — even though some were already members.

"Increasing membership can be an indicator of a successful union," he said.

"Generally speaking, union membership has been declining in recent years. Someone who can achieve increased membership is bucking the trend. It looks good.

"Such increasing membership can give legitimacy to a union's particular objectives in an industrial negotiation or across the industry as a whole."

Union membership could boost influence of branch: Stoljar

Mr Stoljar said union membership numbers could also boost the influence of a particular branch.

"More specifically, if the union is affiliated to the Australian Labor Party [ALP], inflated membership numbers increase the entitlement of that union to delegates to the ALP conference, which in turn leads to greater influence over ALP policy formation, greater influence over membership of powerful ALP committees and, in particular, greater influence over the selection of ALP candidates for political office," he said.

Mr Stoljar said "if false accounting has been practised in order to conceal the payment of membership numbers then the consequences are serious" and could be criminal.

"Other examples of this sort of activity, which will be investigated during the coming hearings, include payments made to the AWU by BMD Constructions Pty Ltd, Winslow Constructors Pty Ltd or its related companies, the Australian Netball Players' Association, the Australian Jockeys' Association and others," Mr Stoljar said.

The hearing before commissioner Dyson Heydon was set down for six days.

Victorian Labor MP Cesar Melhem, the former state secretary of the AWU in Victoria, has been summonsed to appear next week.