"This is just the tip of the iceberg," Ms Garde-Wilson told A Current Affair on Tuesday.

"We first found out about this in 2014 as fact. It took us two years to get the information together to file the proceedings and it wasn't until 2016 that we had them filed. We've been waiting two years before courts for this matter to come to a head."

Her revelations came after Premier Daniel Andrews announced a royal commission into the matter.

Former homicide squad detective Ron Iddles has also revealed that up to 15 senior police officers turned a "blind eye" to the consequences of the arrangement.

The barrister involved cannot be named for legal reasons.

Loading

Ms Garde-Wilson represented dozens of underworld figures and her then boyfriend, convicted murderer Lewis Caine, was gunned down during the bloody gangland war in 2004.

She said some of her own clients would be affected by the fallout from the scandal and she described the implications as "unprecedented".

"These cases all need to be scrutinised one by one... every single one of them needs to be wound back to find out what actually did go on,"  Ms Garde-Wilson told A Current Affair.

"Were they given a fair trial? And what is the consequence as a result if they didn't get a fair trial.

"I don't think it's happened anywhere else in the world, it's unprecedented and it should never have occurred. It pretty much undermines a decade of criminal cases."

Mr Iddles told radio station 3AW on Tuesday morning that he had raised concerns about the use of Informer 3838 after he was asked by former chief commissioner Simon Overland to take a statement from the lawyer in relation to the matters being investigated by the Briars Taskforce, which was looking into the murder of drug dealer Shane Chartres-Abbott.

He took the statement, but never got it signed.

"On return to Victoria I had a further conversation with senior police, including a superintendent, and I said, 'you don't get this. I can tell you now, this will cause a royal commission'," he said.

"I couldn't get that they didn't understand the ramifications of deploying and employing and registering a solicitor."

Mr Iddles said the application to register the lawyer would have been decided on by a committee of senior police.

Retired barrister Peter Faris, QC, who has represented underworld figures including the late Carl Williams and Tony Mokbel, also told ABC Radio National that he believed Police Commissioner Graham Ashton must step down.

"Absolutely [he should step down]... he was involved in this and he's got to stand down," Mr Faris said on Tuesday.

Loading

The former chairman of the National Crime Authority said that everyone was entitled to the due process of the law, which includes having confidence in their lawyer.

"It's absolutely unprecedented, as far as I can see, that a practising busy criminal lawyer is at the same time a registered police informer," Mr Faris said.

Informer 3838 might also be in danger, he added.

Despite calls for his resignation, government ministers have backed Mr Ashton.

Attorney-General Jill Hennessy, who was appointed last week after she spent years as health minister, told ABC Melbourne radio on Tuesday that Mr Ashton had the government's full support.

With Simone Fox Koob, Sumeyya Ilanbey and Adam Cooper