Victoria police spent $4.52 million ($NZ4.7m) on a legal fight to keep secret the use of a criminal defence barrister as an informer during Melbourne's gangland war.
Police in the Australian state said they feared for the lives of the lawyer and her family if identifying information was made public.
The secret arrangement will be the focus of a royal commission next year, which will investigate whether the scandal could taint the convictions of senior gangland criminals, such as Tony Mokbel, drug trafficker Rob Karam and convicted killer Faruk Orman.
On Monday, the High Court lifted suppression orders to reveal the barrister, who cannot be identified, represented Mokbel and other underworld figures while informing against her clients between 2005 and 2009.
The High Court described police's use of the lawyer, known as Informer 3838 or Lawyer X, as "reprehensible conduct" which involved sanctioning "atrocious breaches of the sworn duty of every police officer".
The court also found the defence lawyer had engaged in a "fundamental and appalling breach" of her obligations as a barrister.
Victoria's Director of Public Prosecutions wrote to 20 criminals after the suppression orders were lifted, to let them know their convictions were potentially tainted by the arrangement.
In a statement today, Victoria police confirmed it had spent a total of $4.52m ($NZ4.7m) in legal costs in a bid to keep the arrangement secret.
"Our priority throughout has been the safety of the lawyer and her family who we feared would be murdered if identifying information was released," the statement said.
"We are duty bound to do all we can to keep people safe."
Chief Commissioner Graham Ashton said on Thursday that he was aware of the use of the barrister as an informer during his time at the Office of Police Integrity or OPI.
But he said he had done nothing wrong and would not stand down from the top job.
"I am very confident in my own knowledge and role that I've done nothing wrong in this," he said.
Former homicide squad detective Ron Iddles said on Tuesday that up to 15 senior police officers turned a "blind eye" to the consequences of using the barrister as an informant and he had raised concerns with a superintendent at the time.
"I said, 'you don't get this. I can tell you now, this will cause a royal commission'," Mr Iddles said.
"I just couldn't get that they didn't understand the ramifications of deploying, employing and registering a solicitor."
The Victorian government is yet to appoint two royal commissioners to lead an investigation into the use of the lawyer-turned-informant.
On Thursday, Premier Daniel Andrews said both appointments would be made from interstate to create "appropriate distance" from the Victorian criminal justice system.