In 2004, she helped a Melbourne hitman-turned-police informer make statements that turned him into a witness for the prosecution.

But a royal commission looking at Ms Gobbo's double-life as a criminal barrister and police informer has been told a detective got rid of relevant evidence and kept the extent of the lawyer's role from the courts.

Purana Task Force detective Stuart Bateson on Wednesday admitted he destroyed the key witness' draft and unsigned statement after the man changed his tune about what he knew.

Ms Gobbo had been sceptical of his initial claim as to whether he knew the gangland murder was planned, so went to see him.

The statement changed but Mr Bateson destroyed the draft and did not tell the court about this, counsel assisting the commission Chris Winneke QC said.

"Is it the situation at that stage you had destroyed the draft that had been shown to Ms Gobbo and perhaps changed by Ms Gobbo?" Mr Winneke asked him.

"Yes, that's my understanding," the officer replied.

He said he didn't want the draft floating around for security reasons and because he didn't consider an unsigned statement to be evidence.

"If you get a statement from a person who is a significant crown witness in a murder proceeding, who chops and changes and has one view in which he's saying 'I didn't think it was going to be a murder', then he says, 'I think there was going to be a murder', that's pretty significant, isn't it?" Mr Winneke told him.

But Mr Bateson rejected suggestions he wasn't honest about the statement while giving evidence during the murder case.

"There was no secret that Ms Gobbo had read the statements. We'd certainly discussed that with ... the (Office of Public Prosecutions)," he said.

"I may not have told them that I destroyed it but, certainly when I'm getting asked questions in the witness box ... it's clear that the final product is the only product."

At the time, the court was told the witness knew there was going to be a murder and the man's information implicated other crooks, including prolific gangland killer Carl Williams.

Mr Bateson said the statement given to the court represented "a further slight change to his (witness') belief".

"What we'd have to look to, I suggest Mr Bateson, is the draft copy of the statement," Mr Winneke retorted. "We can't, can we? Because it's been destroyed."

"Yeah," Mr Bateson replied.

Australian Associated Press