Former Victoria Police chief commissioner Christine Nixon has told the Lawyer X royal commission she was never told that Nicola Gobbo was a police informer and cannot be held responsible for her use.

Key points: Former chief commissioner Christine Nixon said she first learnt about Ms Gobbo's role when it became public last year

Former chief commissioner Christine Nixon said she first learnt about Ms Gobbo's role when it became public last year She agreed she should have been told by one of her assistant commissioners, including Simon Overland

She agreed she should have been told by one of her assistant commissioners, including Simon Overland Separately, former deputy commissioner Sir Ken Jones said he believed corrupt prison officers were involved in Carl Williams's murder

Ms Nixon was at the helm of Victoria Police from 2001 to 2009, during the gangland war that was Ms Gobbo's most prolific stint as a double agent.

However, she told the Royal Commission into the Management of Police Informants she was never made aware of Ms Gobbo's secret role.

"I don't recall being told she was a human source — ever," Ms Nixon said.

Asked to explain why senior police officers, including her then-deputy Simon Overland, did not tell her, Ms Nixon said she did not think it was malicious.

"It certainly isn't about ill-will. It certainly isn't about them behaving as they shouldn't be," Ms Nixon said.

She agreed she ought to have been told, but could not explain why she was kept in the dark.

Simon Overland previously told the inquiry he was not told Ms Gobbo's informing breached legal obligations. ( AAP: James Ross )

Counsel assisting the commission Megan Tittensor put to the former chief commissioner that she should have known.

"I did the very best I could," she replied.

But Ms Nixon said she was "not responsible for individual police officer's actions".

"I'm prepared to take broad responsibility for the systems and practices in the organisation, but individual decisions of police officers are theirs."

She told the inquiry she only discovered Ms Gobbo was a police informer when it became public last year.

Deputies should have told me, Ms Nixon says

Over five-and-a-half hours of questioning, Ms Nixon was quizzed about various gangland investigations, but repeatedly stated she had no recollection or knowledge of the specific details of the cases or Ms Gobbo's involvement.

Even when Ms Gobbo was made a witness in a 2009 police prosecution, Ms Nixon said she was not told about her history as a human source.

"I have no absolutely no recollection of people telling me that," Ms Nixon stated.

"Do you think that they should have if they didn't?" Ms Tittensor asked.

"Yes," Ms Nixon said.

Ms Nixon said she had a hands-off approach to investigations, but would be regularly briefed on "major risks" or "potential problems around corruption issues".

Yet the royal commission was told there is no documentation of the briefings as that was not her practice.

"I don't recall it, being told that she was a registered — that she was a human source and, correct, there isn't a record that I can find," Ms Nixon stated.

She said Assistant Commissioner Luke Cornelius would take "some notes".

The inquiry previously heard Assistant Commissioner Cornelius, and the now-Chief Commissioner Graham Ashton, did not keep notes on Ms Gobbo as it was deemed too sensitive.

Nicola Gobbo an 'extraordinary individual', Ms Nixon says

Ms Tittensor asked for Ms Nixon's view of the legal scandal now embroiling the justice system.

"I think you're in an extraordinary circumstance here with an extraordinary individual who comes to the police to become a human source," Ms Nixon said.

She added it was "clearly entirely inappropriate" to use her against her clients.

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Ms Nixon said she kept out of operational matters after being embarrassed by the Milte affair.

Kerry Milte, an ex-police officer and private investigator, was appointed by Ms Nixon to Operation Clarendon, an investigation on organised crime in Victoria.

An Office of Police Integrity report later found Ms Nixon had been duped by Mr Milte, who used the taskforce to further his own career.

He handpicked officers to work alongside, including drug detective Wayne Strawhorn, who was later jailed for drug trafficking.

Former deputy chief says corrupt prison staff involved in Carl Williams's murder

Meanwhile, former Victoria Police deputy commissioner Sir Ken Jones told the inquest he believes compromised or corrupt prison staff were involved in the murder of gangland figure Carl Williams.

Williams was bashed to death by self-styled prison enforcer Matthew Johnson with part of an exercise bike at Barwon Prison in April 2010.

Carl Williams was bashed to death in prison with part of an exercise bike in 2010. ( AAP: Jason South )

In his statement to the Royal Commission into the Management of Police Informants, Sir Ken said Williams had been helping police with their investigation into the executions of gangland informer Terence Hodson and his wife Christine when he was killed.

"The murder of Williams led to the collapse of the prosecution of [former detective] Paul Dale, then charged with the murders of the Hodsons," Sir Ken said.

Mr Dale has consistently maintained his innocence over the murders and a coronial inquest found there was not enough evidence to conclude Mr Dale was behind the Hodsons' deaths.

Sir Ken said he believed Williams's murder was a pre-planned killing and all those involved had not been held to account.

Grainy CCTV footage captured the moment Matthew Johnson attacked a seated Carl Williams from behind with the stem of an exercise bike. ( Supplied )

"I will always believe … that the plan involved corrupted and or compromised staff then serving within Corrections and criminal elements on the outside with much to lose if Williams was ever to give evidence in any criminal trial or Royal Commission," Sir Ken said.

"I recall that there were a number of hypotheses we were considering as well as the obvious link between [the] Carl Williams murder and the prosecution of Paul Dale.

"But all of them shared a common thread; that the murder was planned and involved corrupt actors inside and outside the prison."

Sir Ken Jones described the force's use of Ms Gobbo as "industrial abuse of the criminal justice process". ( Royal Commission into the Management of Police Informants )

In the statement, Sir Ken said he was "very surprised" to see that prison staff had not welded the exercise bike together to prevent a metal part being taken out and used as a weapon — especially given Johnson had allegedly used the same tactic in a previous incident in the prison system.

He also said it was during the investigation into Williams's death that he began to learn about the extent of Ms Gobbo's role as a police informer and the possibility that she had informed on clients she was representing.

Giving evidence at hearings earlier this week, Sir Ken told the commission that Victoria Police's use of Nicola Gobbo was born out of a police force culture that valued loyalty to the boss over the law.

"I thought that was wrong and toxic," Sir Ken said.

At Tuesday's hearings, former chief commissioner Simon Overland insisted he did not know that Ms Gobbo was informing on clients whom she was representing, but conceded "the ethics were f***ed" — a phrase used by Ms Gobbo herself to describe the arrangement.

Read Sir Ken Jones's full statement to the royal commission:

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