Outlaw motorcycle gang members have carried out threats and assaults for the construction, forestry, mining and energy union (CFMEU), Victoria's Assistant Police Commissioner has told an inquiry.

Stephen Fontana told the Royal Commission into Trade Union Governance and Corruption that police have begun several investigations into allegations of violence, intimidation and debt collection carried out by outlaw bikie gang members for the union.

"That group is heavily involved in debt collection and they just go there and stand over people and bypass normal civil process," Assistant Commissioner Fontana said.

He said police had been unable to make any arrests because alleged victims had withdrawn their statements in fear of their own safety.

In a statement CFMEU National Construction Secretary Dave Noonan said the organisation had "zero tolerance for corruption or criminal activity in the union".

Assistant Commissioner Fontana told the hearing in Melbourne that bikies and corrupt union officials knew they could get away with their crimes because the industry was not regulated.

"People have been called to places where they've been assaulted, threatened and a whole range of other things have taken place," he said.

Without better protection laws for whistleblowers, following through with arrests would put people at risk, Assistant Commissioner Fontana told the hearing.

No evidence to back bikie claims: CFMEU

But council for the CFMEU, John Aguis, challenged the Assistant Commissioner to approach the union directly about the allegations.

"Would it not assist you to inform the union you are aware that particular officials may be breaking the law, without naming them?" Mr Aguis asked.

Assistant Commissioner Fontana said it would not assist police "when we've got different investigations going".

Mr Aguis accused Assistant Commission Fontana of waging cultural warfare against the unions.

"You're not about cultural change are you - this is about cultural warfare," he said.

Mr Noonan said Assistant Commissioner Fontana had not provided any evidence to support his claims to the royal commission.

"The Victorian Police have a very important job to do in tackling organised crime and have the full support of the CFMEU," he said in a statement.

"In his appearance today ... Assistant Commissioner Fontana was able to provide no evidence or detail to back up broad allegations of criminality among union officials in the construction industry.

"I again today repeated to the Assistant Commissioner our commitment and willingness to assist where the police have any evidence of corruption or criminal behaviour by union officials," he said.