The Victorian premier says police will investigate two appearances by former union leader John Setka at construction sites over the past two days.
Video emerged of Mr Setka delivering a speech to workers at the Footscray Hospital site and he was also reported to have attended Metro Tunnel construction sites.
Mr Setka stood down as Victorian CFMEU secretary in July following allegations of misconduct within the union reported by Nine newspapers.
The federal government placed the union's construction arm into administration last month, removing 270 union officers from their mostly volunteer positions and a further 11 from their paid jobs.
Victorian Premier Jacinta Allan said Mr Setka's appearance at worksites this week was unacceptable.
"I have been advised that John Setka's attendance on those worksites was unlawful," she said.
"It was unlawful and, as a result, the director general has referred this matter to Victoria Police for further investigation.
"There is no place on Victorian big-build worksites for John Setka. He stood down from the union, as he should have, and there is no place for him on our worksites."
Ms Allan said the director general had also spoken to contractors building large infrastructure projects in Victoria and reminded them that only authorised people are allowed onsite.
"It's not just a concern, it's unacceptable that those processes clearly have not been followed appropriately," she said.
In a statement, Victoria Police said it was awaiting a formal referral.
"Victoria Police has received a verbal briefing from the Police Minister's office about a pending referral regarding a former union official," a Victoria Police spokesperson said.
"Victoria Police will assess the matter once details are provided."
Multiplex, which operates the Footscray Hospital construction site, said it was looking into the matter.
"Multiplex was not aware of and did not authorise any appearance by John Setka on the new Footscray Hospital site. We are investigating how he gained access," it said in a statement.
The Fair Work Ombudsman said it was aware of the allegations former CFMEU officials attended construction sites.
"The Fair Work Ombudsman is making enquiries into these allegations to determine whether any contraventions of the Fair Work Act 2009 (Cth) have occurred," it said.