Victorian Premier Daniel Andrews has announced a royal commission into the use by police of a criminal defence lawyer as an informant during Melbourne's bloody gangland war.

Key points: The royal commission will investigate how many criminal convictions may have been affected

The royal commission will investigate how many criminal convictions may have been affected The Premier said he had been assured police had since changed their practices

The Premier said he had been assured police had since changed their practices The commission will consider if changes need to be made to the management of informants

The lawyer represented notorious underworld figure Tony Mokbel and six of his associates, while acting as a paid police informant against some of her clients.

The announcement of a royal commission followed the lifting of suppression orders this morning on a case decided in the High Court last month involving Victoria Police, the lawyer-turned-informant and the Victorian Director of Public Prosecutions (DPP).

Court documents revealed the lawyer claimed to have helped police in as many as 386 cases, including that of Mokbel, who in 2012 was sentenced to 30 years' jail for masterminding a sophisticated drug cartel known as The Company.

She also provided information on the importation in 2007 of 4.4 tonnes of ecstasy inside tomato tins shipped from Italy. Mokbel associate Rob Karam is serving a sentence of more than 35 years' jail for his part in the case.

The revelations open up the prospect of criminals having their convictions overturned and walking free from prison, and potentially expose the state to claims for compensation from those affected by the lawyer's work as an informant.

The High Court judgement said by acting as an informant against her clients, the lawyer committed "fundamental and appalling breaches" of her obligations.

The High Court condemned Victorian police, saying they were guilty of "reprehensible conduct" in encouraging the barrister to give up information about the accused and were involved in sanctioning "atrocious breaches of the sworn duty of every police officer".

"As a result, the prosecution of each convicted person was corrupted in a manner which debased fundamental premises of the criminal justice system," the court said.

The barrister represented Tony Mokbel and six of his associates. ( AAP: Julian Smith )

Mr Andrews said Victoria Police had given the Government assurances that its practices had changed since the barrister was recruited as an informant, and that a 2015 report by the Independent Broad-based Anti-corruption Commission had found that no unlawful conduct had occurred.

But he said the Victorian community had a right to know what had happened in the case, and to be assured that such practices had been stamped out.

"Given there's a prospect of some very well-known individuals walking free, we think it's appropriate to have this highest and most formal type of inquiry … to give us the answers we need and the certainty that this could never happen again," Mr Andrews said.

Mr Andrews said the safety of the lawyer and potentially her children were at risk.

Victoria's Director of Public Prosecutions, Kerri Judd QC, has written to 20 criminals whose convictions may have been affected by the force's use of the barrister as an informant.

"Other cases are being assessed. If appropriate, I will also write to those affected individuals," Ms Judd said in a brief statement.

Sorry, this video has expired Graham Ashton said police protocols for informants have changed since the incident

Police say informant policy changed since incident

Victoria Police Chief Commissioner Graham Ashton acknowledged that the High Court had determined that police's use of the lawyer as an informant between 2005 and 2009 was not appropriate and said it had set up a taskforce to deal with any legal implications flowing from the case.

He said since 2009 the force had made changes to the way it managed informants, which would prevent the same thing happening today.

Chief Commissioner Ashton said he believed two Commonwealth cases could be affected, in addition to the 20 cases identified by the Office of Public Prosecutions.

He said police would fully cooperate with the royal commission, but said he hoped the royal commission would take into account the context of the "desperate and dangerous" time at which the lawyer was recruited as an informant.

"Melbourne was in the grip of what is now rightly known as the gangland wars," he said.

"Over the preceding 12 months numerous people had been murdered, some in very public locations, and high-profile criminals were vying for control of drug operations that were inflicting serious harm on the Victorian community.

"The risk to the community at this time was significant."

Chief Commissioner Ashton said that he believed the current and retired officers who managed the informant had acted "in good faith," adding "they can expect to have my continuing support".

He defended police's legal efforts to suppress the arrangement, adding that police remained concerned for the safety of the lawyer.

"At all times when handling these matters our absolute concern has been for the protection of the lawyer and their family, who Victoria Police believed would be murdered if this information was released," he said.

"Nothing is more important to Victoria Police than the protection of human life."

Criminal compensation claims possible

Solicitor Zarah Garde-Wilson said she had multiple clients who would be watching the legal developments closely.

"There'd only be a handful of people who are convicted having [pleaded] not guilty who would be filing appeals," she said.

"However, there would be far more people who have [pleaded] guilty, or who have served their sentences, who would be looking for petitions of mercy, or to have their convictions overturned.

"And there are multiple people who may have been unfairly convicted, and have suffered significant financial loss, who would be looking for compensation."

Retired barrister Peter Faris QC, who represented Carl Williams and Tony Mokbel, told ABC Statewide Drive the affair was "completely disgraceful and must never, ever happen again".

"I worked as a criminal lawyer for 55 years. The first rule was everything your client tells you is totally confidential and you take it to the grave.

"To have a paid police informer who was a lawyer conveying a client's information to the police is the biggest breach you could imagine of that principle."

Former Victoria Police assistant commissioner Noel Ashby said the impact of the revelations would be felt for years to come.

Noel Ashby said the revelations would harm the credibility of police evidence in the future. ( ABC News: James Oaten )

"This will be raised before juries into the future," he said.

"Lawyers will say, 'Can you trust police? Can you trust the evidence they're putting forward?'"

Earlier, Mr Andrews was flanked by Police Minister Lisa Neville and his new Attorney-General, Jill Hennessy, as he announced the royal commission.

He said the royal commission will first determine how many cases were impacted by the use of the barrister as an informant.

It will also consider what changes may need to be made to the management of informants by police.

The royal commission will also make recommendations on how to deal with those whose criminal cases may have been impacted by the use of the barrister as an informant, including the status of their convictions and any claims for compensation.

Mr Andrews said the terms of reference for the royal commission would be determined in consultation with the royal commissioner or royal commissioners, who are yet to be appointed.

The royal commission will have a budget of $7.5 million and will provide an interim report by July 1, 2019, and a final report by December 1, 2019.