SYDNEY (AFP) - Hundreds of criminal convictions - including those meted out to some of Australia's most notorious gangland figures - hang the balance, after their defence lawyer was revealed as a police informant on Monday (Dec 3).

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 on 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 judgment.

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

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 against their convictions.

Victoria Premier Daniel Andrews on Monday 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".