Victorian protective services officers (PSOs) have unlawfully arrested hundreds of people over eight years for failing to appear in court, Victoria Police has revealed.

Key points: PSOs patrol train stations across Melbourne and regional Victoria

PSOs patrol train stations across Melbourne and regional Victoria Legal advice has revealed arrests for people on failing to appear in court warrants are unlawful

Legal advice has revealed arrests for people on failing to appear in court warrants are unlawful Victoria Police officers are now "supporting" PSOs on the job

Last year the armed guards, which were introduced in 2011 and began patrolling Victorian train stations in 2014, arrested 500 people.

Victoria Police has confirmed new legal advice has brought into question the arrests of people on warrants for failing to appear at court only.

"The issue we have identified relates to a very small percentage of the total arrests made by PSOs, as few as one or two a day," Victoria Police regional operations Acting Deputy Commissioner Bob Hill said.

"It was an issue created in the drafting of the original legislation, which supported the inception of the PSO Transit Policing Model in 2011."

PSOs routinely arrest people, but Victoria Police said it has found the powers to arrest people on warrants for failing to appear in court belong solely to police officers.

"It's important to appreciate, the majority of the 500 arrest warrants executed last year involved persons being lawfully arrested by PSOs who witnessed the commission of other criminal offending," Acting Deputy Commissioner Hill said.

"Execution of the fail to appear warrants was subsequent to the arrest.

"The issue we are dealing with is limited to the rare occasions when PSOs are making arrests solely because they have identified a person as being wanted on a warrant, and some of those warrants are currently addressed to exclude PSOs."

Police officers have now been sent to shadow PSO teams at railway stations, where the police will remain until the Government makes legislative changes.

"We are working on a fix with the State Government to ensure our PSOs continue to have the powers they need," Acting Deputy Commissioner Hill said.

"In the interim, we have additional transit police deployed supporting their PSO colleagues on the transport system."

A Protective Services officer is patrolling a carriage of a Melbourne train. ( ABC News )

The Andrews Labor Government has committed to amending the legislation as soon as possible, and has been quick to place the blame at the former Liberal government's door.

"The Liberals botched the drafting of this legislation and we're now working with Victoria Police to fix it as a matter of priority to ensure our protective services officers have the powers they need to keep the community safe," a Government spokesperson said.

"Victoria Police has clear legal advice that the overwhelming majority of arrests made by PSOs have been lawful.

"PSOs will continue to be out every night in force keeping Victorians safe."

PSO powers were extended in 2017 allowing them to search and arrest people who breached their parole.

They were also given the power to request names and addresses from people who witnessed a crime, and to respond to incidents in and around the train stations they patrol.

"Let me be very clear on this point: if you are wanted on warrant and engaged by our PSOs and police, you will be arrested and brought to justice," Acting Deputy Commissioner Hill said.

"Our PSOs do an exceptional job keeping people safe on our public transport system.

"Every day they step forward, protecting the community and making arrests, in good faith and in accordance with their training."

Police association says PSOs were working within their training

PSOs work at 212 Melbourne train stations and at Bendigo, Traralgon, Ballarat and Geelong in regional Victoria.

The Victorian Police Association has backed the PSOs, explaining the officers were acting within their training.

"Our PSO members have acted in good faith with the best community interests, relying on the legislative framework that was set up for them," Police Association Victoria secretary Wayne Gatt said.

"All of the people arrested have been subsequently handed over to police by PSOs and brought before the courts. That is a good thing for community safety.

"In the overwhelming majority of cases, PSOs will have detained people for substantive offences using arrest powers derived from other laws.

"In relative terms it requires a very minor legislative amendment, something we do not anticipate will take long to achieve or be controversial in any way."