Lawyer-turned-police informer Nicola Gobbo bragged about secretly editing all the statements made by a gangland hitman whose evidence was pivotal in numerous gangland trials, police documents have revealed.

Key points: Ms Gobbo told her handlers a police officer was with her when she edited all the statements of a gangland hitman

Ms Gobbo told her handlers a police officer was with her when she edited all the statements of a gangland hitman The witness made more than a dozen statements that helped secure several convictions

The witness made more than a dozen statements that helped secure several convictions The former head of the Purana Taskforce denied any knowledge of the plan

The Royal Commission into Management of Police Informants was told the transcript of the meeting Ms Gobbo had with her handlers in August 2008 only emerged on Monday morning.

In the meeting, Ms Gobbo said she had secretly gone to the office of the Purana Taskforce and edited all the statements of a key witness, who had turned against his gangland bosses.

"I went to Purana secretly and edited all of his statements," Ms Gobbo said.

"I corrected them but no one ever knows about that, that would never come out, even [he] doesn't know I did," the transcript showed Ms Gobbo said.

"He could never reveal it because he doesn't know about it."

The witness, who cannot be named, made more than a dozen statements that were crucial in bringing about gangland convictions, including that of murderer Carl Williams.

The revelation has cast further doubt on many gangland convictions.

Evidence of the witness whose statements were edited contributed to Carl Williams pleading guilty to three murders. ( Pool photo )

Ms Gobbo told her handlers she went to the office with a police officer who was not connected with the witness, to avoid the risk of him being called to give evidence that could reveal what they had done.

"They were very good the way they did it because the detective that I did it with is not a witness, so it can never come out with the people just telling the truth," she said.

"It was a well-thought-out plan," she bragged.

Ms Gobbo said she believed it was the idea of the former head of the Purana Taskforce, Jim O'Brien, or a senior handler.

Counsel assisting the royal commission Megan Tittensor put that to Mr O'Brien, who denied any knowledge of the plan.

Asked if it amounted to perverting the course of justice, Mr O'Brien said he could not say, but agreed it should never have happened.

However, he voiced some scepticism over Ms Gobbo's account.

"Hypothetically speaking yes it's wrong, clearly it's wrong, but not knowing the facts, it's hard to say," he told the royal commission.

Mr O'Brien said the handlers should have reported what she had told them up the chain.

The inquiry heard the witness statements were very significant for the taskforce, and would lead to Carl Williams pleading guilty to three murders.

The evidence of the witness was used in the trial of Faruk Orman, who walked free in July over the Lawyer X scandal. ( AAP: David Crosling )

Williams's conviction was a celebrated moment in Melbourne's gangland war, bringing accolades for the Purana Taskforce from then-chief commissioner Christine Nixon and the Victorian Government.

The evidence of the witness was used in the trial of Faruk Orman, who was later convicted of being the getaway driver in the 2002 murder of Victor Peirce.

His conviction was the first to be quashed in the wake of the Lawyer X scandal.

The lawyer for Victoria Police, Saul Holt SC, told the commission the transcript of the meeting was made from a corrupted tape recording, which had caused a delay in providing it to the royal commission.