The current and former Victoria Police commissioners who knew about and authorised the recruitment of Lawyer X could face criminal charges, according to the state's former chief crown prosecutor.

Lawyer X, who was unmasked as high-profile criminal barrister Nicola Gobbo, was a registered police informer through the 1990s and during Victoria's gangland war.

Her clients were a who's who of the underbelly world, including infamous gangsters like Carl Williams and Tony Mokbel.

Infamous mobsters Carl Williams and Tony Mokbel were represented by Ms Gobbo. ( Four Corners: Georgina Piper )

Ms Gobbo's actions, sanctioned by the police hierarchy, have now placed Victoria Police at the centre of a royal commission that is calling into question notable criminal convictions and the conduct of some of the state's most senior police.

They include current Victoria Police Chief Commissioner Graham Ashton and former commissioner Simon Overland.

Former Victorian chief crown prosecutor Gavin Silbert QC believes that if Ms Gobbo is charged after the royal commission, it is likely police officers would also have to be charged.

"They were all perverting the course of justice," he said.

"The upper hierarchy of Victoria Police has to take complete responsibility.

"Those who knew and sanctioned what was happening were guilty of terrible breaches of duty and extraordinarily unethical behaviour."

Former Victorian chief crown prosecutor Gavin Silbert QC said if Ms Gobbo is charged, it is likely some police will be too. ( Four Corners: Georgina Piper )

Mr Silbert, who retired in 2018, said the evidence revealed so far seems to point to Commissioner Ashton and Mr Overland as "being parties to what's gone on".

He said it is possible some senior police officers could face jail time.

Lawyer Ruth Parker, who had her client's murder conviction quashed because of the scandal, said Victoria Police were fully aware they were undermining the justice system.

"Anybody who thinks that registering a criminal defence barrister as a human source, using them against her clients, facilitating dishonesty in the court system, facilitating perjury, and covering up the fact that it had happened, that is the very definition of corruption," she said.

How were the top cops involved?

Throughout the early 2000s, two of Melbourne's most notorious crime gangs waged a bloody fight for control of the city's lucrative drug trade.

Police were under pressure to rein in the violence as the bodies piled up.

As the gangland figures rolled up to court, Ms Gobbo was often at their side representing them.

She also became the secret weapon for the police in the underworld war, betraying her clients to help police secure convictions.

At the time of Ms Gobbo's last registration as an informer in 2005, Commissioner Ashton was at the Office of Police Integrity.

Commissioner Ashton and then-assistant commissioner Overland were on the steering committee overseeing the investigation into the murder of another police informer, Terry Hodson and his wife Christine.

The involvement of Commissioner Graham Ashton and the former commissioner Simon Overland has been heavily criticised.

Despite her handlers' growing concerns about her safety, Victoria Police made the extraordinary decision to have Ms Gobbo wear a wire to record a conversation with former drug squad detective Paul Dale who was the main murder suspect.

After recording the conversation, Ms Gobbo initially said she was prepared to be a witness against him.

The royal commission has heard that the steering committee ignored warnings that this could put Ms Gobbo at grave risk.

Former superintendent Ian Baker was with the Victorian force for 41 years and is critical of the decision to recruit Ms Gobbo as an informer and subsequently as a witness.

"I will say that whoever made that decision was extremely naive and it showed a complete lack of operational experience," he said.

The charges against Mr Dale over the Hodson murders were later dropped after the murder in prison of drug lord Carl Williams, who was to be a witness against Mr Dale.

Dale recently told the royal commission he had been set up, saying, "I truly believe they've committed a criminal offence…perverted the course of justice".

Two secret reviews into the handling of Ms Gobbo delivered highly critical assessments of police conduct.

While Mr Overland left the police in 2011, Mr Silbert said he is surprised Commissioner Ashton has managed to stay in his job.

"I don't know how he's lasted so long," he said.

"Public perceptions of the police force have been very damaged by this, and as the High Court has said, it goes to the fundamental foundations of the whole of the criminal justice system.

"How that's repaired, I don't know. But one would have thought anyone in the hierarchy who sanctioned this should have gone."

'Culture starts at the top': former officers

Ian Baker and Charlie Bezzina both served in the Victorian Police force during the gangland war. ( Four Corners: Georgina Piper )

Both Chief Commissioner Ashton and Mr Overland came to Victoria Police from the Australian Federal Police and had a different approach to policing that rubbed badly with the old guard.

The investigating detective on the Hodson murders, Charlie Bezzina, believes that was part of the problem.

"These are federal police officers. They don't have the experience of coal-face policing, because federal policing is so different from state policing. Yet here they are calling the shots," Mr Bezzina said.

Mr Baker said responsibility for what has become the biggest legal crisis in Australian history rests with the senior police leadership.

"Culture starts at the top. The head of the fish rots first," he said.

"This is going to be a very, very dark time in the history of the Victoria Police force.

"It's going to take some time to rebuild the trust of the community, to bring back ethics into the job."

Commissioner Ashton and Mr Overland have yet to appear before the royal commission and both declined to be interviewed by Four Corners.

Victoria Police has been criticised for frustrating the workings of the royal commission.

Commissioner Margaret McMurdo has even warned police they face prosecution for failing to produce documents and witness statements requested by the inquiry.

Mr Silbert said the police have been deliberately slow.

"Documents have been dribbled forth to the commission, always late and not with adequate time for those affected to cross-examine on them," he said.

"There's obviously been a concerted attempt to stymie the commission as much as been possible."

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More gangland figures may walk free

Reviews into dozens of convictions are now underway and in what could be a bitter irony, the very methods used to put so many criminals behind bars may be the trigger for their release.

Already one of those gangland figures has walked free.

Faruk Orman served 12 years for murder.

His conviction was based on tainted evidence created with the assistance of information provided by Ms Gobbo.

Mr Orman's lawyer Ruth Parker told Four Corners she believes the police commanders who sanctioned the decision to recruit and use Ms Gobbo need to take full responsibility.

"But that can only happen when the people whose personal interests are divided from the interests of the organisation," she said.

"How you can have an organisation run by a person such as Graham Ashton in circumstances where he is well and truly in the spotlight, potentially having committed criminal offences himself."

Criminal lawyers Zarah Garde-Wilson and Ruth Parker have criticised the police who sanctioned Ms Gobbo's actions. ( Four Corners: Georgina Piper )

Zarah Garde-Wilson was another young defence lawyer who represented well-known underworld figures at the time of the gangland war.

Ms Garde-Wilson told Four Corners that Victoria Police should be held accountable for the damage that has been done to the justice system.

"But for their actions this would not have occurred. When she walked in the door and said, 'I want to be an informer', they should've turned around and said, 'Not possible, thanks very much'," she said.

Ms Garde-Wilson's client, Rob Karam, was convicted of drug trafficking and sentenced to 37 years in jail.

He is just one of the underworld figures who will be appealing. Tony Mokbel is another.

Mokbel's lawyer, Richard Maidment QC, told the royal commission that he believed the actions of Victoria Police were criminal.

"This is not just a case of impropriety. There is criminal conduct there and that involves aiding and abetting [Ms] Gobbo in perverting the course of justice," he said.

"This is a situation that is their own making. Nothing like this has ever occurred to my knowledge."