Victoria Police Chief Commissioner Graham Ashton has denied he was involved in a deliberate cover-up over the prolific use of gangland lawyer Nicola Gobbo as a police informer.

Key points: Past and present police chiefs are facing the Lawyer X royal commission this week

Past and present police chiefs are facing the Lawyer X royal commission this week A lawyer for the commission suggested the current police chief tried to "keep a lid on" information about her use

A lawyer for the commission suggested the current police chief tried to "keep a lid on" information about her use Graham Ashton denied there was an "attempt to conceal" information, and that he stopped a police integrity probe on the orders of an assistant commissioner

Commissioner Ashton is the first of the top brass to take the stand at the royal commission that is scrutinising Ms Gobbo's use as a source.

He was quickly asked if he had tried to stop the information getting out.

He told the inquiry he had known Ms Gobbo was a source, but was shocked to learn of the full extent of her use in 2011.

"I was aware she was a human source before that, but certainly not aware of the extent of it until that period of time," Commissioner Ashton said.

He said he asked for the Australian Federal Police and the Office of Public Prosecutions to be told, but did not realise that information was not passed on.

At the time, Tony Mokbel's drug trial and the infamous tomato tins drug smuggling case were before the courts.

Counsel assisting the commission Chris Winneke accused Commissioner Ashton of trying to "keep a lid" on the information so people did not find out.

"If you were serious that you were concerned about appropriate disclosure being made, if that was your real concern, then appropriate disclosure would have been made rather than an attempt to conceal, I suggest, what has occurred since you became aware of this information," Mr Winneke challenged.

"There was no attempt to conceal," Commissioner Ashton responded.

He also told the inquiry he had not known about Ms Gobbo's previous registrations as a source in 1995 and 1999 until recently.

Evidence before the inquiry suggests the police chief was told about her role as a double agent in 2006.

But Commissioner Ashton said he first learnt Ms Gobbo was a police informer in 2007, when he was the deputy commissioner with the Office of Police Integrity (OPI), the body set up to stamp out corruption.

The inquiry is investigating how Victoria Police used barrister Nicola Gobbo as a human source. ( ABC News, file photo )

Gobbo involved with lots of people on different sides

As the OPI deputy commissioner, Commissioner Ashton was overseeing two police taskforces, including Taskforce Petra, which was investigating the murder of police informer Terence Hodson and his wife Christine.

The inquiry was told Commissioner Ashton had also been looking into the affairs of Ms Gobbo in 2005, including her possible involvement in the leaking of a police file that revealed Mr Hodson was a police informer.

What he found, Commissioner Ashton agreed, painted a "troubling picture" of Ms Gobbo, but he did not question why police would use her as an informer.

Mr Winneke said it was clear she was "someone who would get herself involved with lots of people on different sides" and that she was suggesting clients roll on one another.

When pressed, Commissioner Ashton said it was wrong for police to use her to get criminals to roll on each other — if police did not disclose that information during court proceedings.

"It would be wrong to permit that to occur and then not to disclose that, yes," he said.

Commissioner Ashton agreed that the royal commission had uncovered several instances where that occurred.

"It's troubling, yes," he told the inquiry.

He agreed it could amount to a perversion of the course of justice in certain circumstances.

When the Lawyer X scandal first broke, Commissioner Ashton defended the use of Ms Gobbo, saying the gangland war was a "desperate and dangerous" time for Melbourne.

Ashton rejects claim he was pressured to drop probe

Mr Ashton also denied he stopped an OPI investigation of Ms Gobbo on the orders of then-assistant commissioner Simon Overland in 2006.

Evidence before the inquiry suggests Mr Overland intervened to stop the OPI investigation, which was related to the Hodsons' murders.

Diary notes made by police handlers said they raised the matter with Mr Overland, because they were concerned Ms Gobbo would be exposed if the investigation went ahead.

The notes say it was agreed Mr Overland would speak to Mr Ashton to advise him not to pursue the investigation.

Mr Ashton's own diary entry from the time also notes a meeting took place with Mr Overland and then-assistant Commissioner Luke Cornelius.

But on the stand, Mr Ashton firmly denied being told of Ms Gobbo's informer status at the meeting and refuted the claim he had kowtowed to police demands.

However, Mr Ashton could not explain why the planned OPI examination was dropped, and no statement was taken from Ms Gobbo.

"I haven't been able to work out why the hearing didn't go ahead," Mr Ashton told the commission.

He also said it was not in his power to stop a coercive examination.

Ms Ashton also said he did not think Ms Gobbo's role was relevant to his work as a corruption investigator.

"Yet here we are ten years later investigating just that," Mr Winneke replied.