Judge Ian Gray said Williams' murder had already been one of the most investigated deaths in Victoria and an inquest would be an unnecessary duplication of several other investigations which had already been conducted.

Judge Gray said he could see no legitimate reason for holding an inquest.

The coroner said the only reason for an inquest would be to investigate management issues at Barwon Prison, and that he could find no systemic defects which had not already been addressed by other agencies and government bodies.

Williams' family had been pushing for an inquest to investigate whether guards failed to properly monitor Williams when he was killed in the prison’s high-security Acacia section in April, 2010.

Williams' former wife, Roberta, and his father, George, are suing the state over Carl's death on the grounds of negligence.

The Williams family sent a submission to the State Coroner wanting to know why CCTV cameras inside the Acacia unit were not being monitored at the time Williams was attacked and killed by Matthew Johnson, and why it took security staff so long to discover his body.

The family was also concerned over why Williams, 39, had been put in the same unit as Johnson, a notoriously violent inmate who hated police informers.

State Ombudsman George Brouwer found in 2012 that monitoring systems were so poor inside the unit it took prison guards 27 minutes to find Williams's body, even though he was a murderer and a police informer at the time.

Mr Brouwer said officials had been repeatedly warned that Johnson was a threat to Williams.

When jailing Johnson for 32 years in 2011 after he had been found guilty by a Supreme Court jury of murdering Williams, Justice Lex Lasry said Williams had been helping police with their investigation into the murders of police informer Terrence Hodson and his wife Christine at Kew in May 2004.

The judge said Williams had made three statements implicating himself, hitman Rodney Collins and former drug squad detective Paul Dale, in the Hodson murders.

As a result of these statements, Dale and Collins were charged with murder.

Carl Williams told his father that he had explained to Johnson that he was talking to investigators about police corruption, Justice Lasry said.

Williams wanted Johnson to believe he was not going to give evidence about other prisoners.

The judge said Williams and Johnson then spent a lot of time together in the Acacia unit.

''How the prison authorities permitted that to happen is beyond me,'' Justice Lasry said.

''On any view, you [Johnson] were a threat to his [Williams’] welfare as long as he was on the record as a Crown witness against Dale and Collins.''

On april 19, 2010, at about 12.48pm, Johnson approached Williams from behind as he sat at a table reading a newspaper, and hit him with the seat post from an exercise bike.

''The CCTV shows clearly that Williams had no idea you were there and about to attack him,'' the judge said. ''You struck Williams once to the right hand side of the head causing him to fall off his chair. ''Once on the ground you struck him another seven times to ensure that he was dead.''

After Williams was murdered, the charges against Dale and Collins were dropped.

Senior Sergeant Stuart Bailey, the police officer who compiled the brief of evidence for the court, said the case had been thoroughly investigated and he supported the coroner’s decision.

Senior Sergeant Bailey said the Williams’ family understood the coroner’s reasoning but now had the option of appealing the decision to the Supreme Court.

Judge Gray said part of the Williams’ family submission calling for an inquest included:

• How was it that no protection was given to Mr Williams after a Herald Sun front page article reporting that police were paying private schooling fees for Mr Williams’ daughter?

• How was it that Mr Johnson had a copy of Mr Williams’ statement implicating Paul Dale in the murder of Terrence Hodson, a police informant at the time, on his computer two days before the murder?

• How was it that Mr Johnson had access to a lethal weapon in the form of part of an exercise bike, in particular within the circumstances where there were allegations that Mr Johnson had assaulted a prisoner in a similar way before?

• Whether there was a relevant connection between Mr Johnson and Mr Dale, especially in the circumstances where witnesses implicating Mr Dale, a police officer, had been murdered (Mr Williams had also implicated Mr Dale).

• When Mr Williams was removed from prison to speak to Victoria Police, what mechanisms (if any) were in place to prevent other inmates from learning about the assistance?

While acknowledging that Carl Williams was a much-loved family member, Judge Gray said he was satisfied that ‘‘the identity of the deceased, cause and circumstances of the death can be established without holding an inquest’’.

An inquest into the murders of the Hodsons is expected to begin in May.

Williams’ former wife Roberta and his father George were in court for Judge Gray’s ruling but refused to comment when leaving.