Australia's High Court blasted police as being "guilty of reprehensible conduct" after it was revealed a high-profile Melbourne barrister had for years been giving information to Victoria Police

Hundreds of criminal convictions -- including those meted out to some of Australia's most notorious gangland figures -- hang in the balance, after their defence lawyer was revealed as a police informant Monday.

Court documents disclosed that a high-profile Melbourne barrister had for years been giving information to Victoria Police, while representing clients like crime boss Tony Mokbel and six of his associates.

The revelation came after court injunctions were lifted Monday, blowing the lid off one of the biggest legal scandals to hit the country in years and prompting the authorities to immediately announce a public inquiry.

In a secret court battle spanning more than two years, police had tried to stop state prosecutors from informing the seven people about their barrister's supergrass role.

They were finally knocked back by Australia's highest court last month.

The High Court blasted police as being "guilty of reprehensible conduct" and said they were "involved in sanctioning atrocious breaches of the sworn duty of every police officer".

"The prosecution of each convicted person was corrupted in a manner which debased fundamental premises of the criminal justice system," the High Court added in a scathing judgement.

The lawyer -- given the code name 3838 by police when she was working as an informant between 2005-2009 -- kept in daily contact with her handlers, who passed on information provided by her to various police taskforces, court documents stated.

While the courts only mentioned Mokbel and his associates, more than 600 cases could be tainted, according to Melbourne's Herald Sun.

Mokbel was jailed in 2012 for a minimum of 22 years for masterminding a drug trafficking empire.

The High Court decision means criminals would be notified that their barrister, who still cannot be named, was a police informant, thus possibly affecting their cases.

It would mean these criminals could then appeal their convictions.

- 'Gangland wars' -

Victoria's Director of Public Prosecutions Kerri Judd said she has written to 20 individuals about their cases, and was assessing others.

"If appropriate, I will also write to those affected individuals," she said in a statement.

Victoria Premier Daniel Andrews meanwhile announced a royal commission into the scandal, saying the public had a "right to know that every part of the justice system acts fairly and lawfully at all times".

Victoria Police Chief Commissioner Graham Ashton said the force changed how it managed informants in 2009, so such incidents cannot be repeated today.

Ashton said while police would cooperate with the probe, what was happening in the underworld at that time should be taken into account.

"Melbourne was in the grip of what is now rightly known as the gangland wars," he told reporters. "The risk to the community at this time was significant."

Legal advocacy group The Human Rights Law Centre said an independent investigative body should also be established to probe all allegations of police misconduct.

"This case touches the highest levels of Victoria Police. While this is an extreme case it's not an isolated example of police misconduct," said the centre's Ruth Barson in a statement.