Hundreds of criminal convictions may be at risk after it was revealed the defence barrister involved in the cases was a police informant.

Court documents have revealed a high-profile lawyer in Melbourne had been giving information to police while defending clients including crime boss Tony Mokbel and six of his associates.

The details emerged after court injunctions were lifted, revealing one of the biggest legal scandals to hit Australia in years.

Authorities have announced an inquiry will take place.

In a judgment, the High Court said police were "guilty of reprehensible conduct" and added: "The prosecution of each convicted person was corrupted in a manner which debased fundamental premises of the criminal justice system."


Image: The cases of dozens of people could be reviewed, including Tony Mokbel

The lawyer in question had the code name 3838 when she worked with police between 2005 and 2009 but cannot be named at this time.

According to ABC Australia, the informant gave police information about other clients she was representing, and those clients then testified against Mokbel and the associates.

It was decided if the information was released, her life would be at risk.

Mokbel was jailed for 30 years in 2012 for his role in a drug syndicate called The Company.

The Herald Sun newspaper suggests more than 600 convictions could hang in the balance now, but only Mokbel and his associates were mentioned by the court.

Criminals who had her as a barrister will be informed and could potentially appeal their convictions.

The region's director of public prosecutions has written to 20 individuals about their cases and will assess if any others need to be informed.

Victoria Police chief commissioner Graham Ashton said the way police handle informants had changed since 2009 and this would not be repeated.