These men and women are now in their 40s and 50s, but whenever they see the monster we will call Bill, they feel like they're vulnerable children all over again.

They sat together, holding hands or patting each other on the shoulder as they listened to the evidence; gut-churning details about children as young as five being raped and sodomised by a monster whose job it was to serve and protect.

These days, Bill is an overweight, white-haired man with chronic osteoarthritis in his knees and shoulders. He shuffles when he walks, and he squints through thick spectacles. He looks harmless, and older than his 66 years.

But looks can be deceiving.

Bill, a former policeman, is one of Victoria's worst sexual predators. This week he was jailed for 19 years, with a minimum of 15, for raping and abusing nine children over 16 years.

It can now be revealed that those crimes are just a fraction of his habitual sexual offending against children. Bill was as young as 17 when he started attacking children; his most recent offences were in 2011.

Bill joined the Victoria Police academy straight out of school, in 1968. By then, he had already abused a boy, forcing the child to give him oral sex before giving him a Kit Kat.

Threats and rewards were his methods; after he raped one weeping boy, Bill told the child that if he told anyone, he would kill the boy's mother and sisters.

He seemed to enjoy using his power as a police officer to control his victims.

One woman told the Victorian County Court last week that Bill had held his police-issue revolver to her head as he raped her as a child. He assaulted another child in a room at a police station, and another in the back of a police divisional van.

"You clothed yourself in the respectability of the uniform of Victoria Police," County Court judge John Carmody told him on Tuesday.

"And [you] betrayed that respect by the most base of human behaviour – offending sexually against vulnerable children."

A former colleague of Bill's, who worked with him at a Melbourne city police station, told Fairfax on Tuesday there had been suspicions about Bill's behaviour at that station in the early 1970s.

"His name disappeared from the roster overnight with the explanation he had been transferred out," the former police officer said.

"Rumour then suggested he may have been involved in nefarious sexual activity while on duty."

The former colleague has now passed on those concerns to Crime Stoppers, but it's not known whether any of Bill's colleagues reported their concerns to their supervisors at the time.

But even when Victoria Police received concrete information that Bill had interfered with a child, it failed to act on that information – beyond quietly forcing him to resign.

This failure to act, we now know, exposed a dozen more children to assault and rape at the hands of this predator.

Bill – who admits he is a paedophile – committed a similar reign of terror in NSW, after leaving Victoria in 1988, abusing another seven children between 2005 and 2011.

Bill's career with Victoria Police came undone in 1979. A young boy had been riding his horse and throwing rocks at passing trains in a Victorian country town, much to the annoyance of passersby.

Someone reported the boy to police, and a few days later the boy's mother took him to the local police station, where he was reprimanded by the sergeant and sent home.

A couple of days later, out of the blue, Bill turned up, dressed in his police uniform.

Somehow, he got the boy alone in a bedroom.

Once there, Bill stuck his hand inside the boy's shorts, yanked hard on the child's penis, and told him he was going to "rip it off", before leaving.

The child immediately told his family what had happened and his mother went to the local police station to talk to the officer in charge.

The next day, "a number" of police turned up at the family home to question them about the assault.

The mother told them she no longer wanted to take the matter any further but, in any event, police said, Bill would no longer be in the force and "would be moved away".

He was not prosecuted for the assault at the time, but last week he pleaded guilty to assaulting that boy – part of a raft of 18 offences against nine children that Judge Carmody described as being "of the most serious nature".

A spokeswoman for Victoria Police would not comment on the case or answer questions about what police knew about Bill in the 1970s.

"As the matter is still before the court it would be inappropriate to comment," she said. "A 28-day appeal period applies where the accused can appeal the severity of their sentence."

The last time Bill was sentenced, in the Sydney District Court in 2013, it was the Director of Public Prosecutions that appealed, arguing his sentence for 20 crimes against seven boys was inadequate.

The Court of Appeal agreed, increasing his sentence to 17 years with a non-parole period of 12 years, for abuses committed between January 2005 and December 2011 in Sydney.

The court was told that Bill had befriended the boys' families, and insisted the children call him "Uncle".

He showered them with alcohol, and expensive gifts including trail bikes, PlayStation games, radio-controlled cars, pornographic magazines, clothing and a gym set.

When one boy turned 16, Bill abandoned all contact with him. He was aged between 55 and 62 when he preyed on the boys; his victims were aged between nine and 16.

There are now 16 known victims, who were abused between 1967 and 1983, and between 2005 and 2012.

Bill's activities between 1983 and 2005 are unknown.

Despite the wishes of three of his victims, who believe more victims would come forward if Bill's real name is made public, Fairfax is unable to name him at the insistence of the County Court, to avoid the possible identification of other victims.

The Royal Commission into Institutional Responses to Child Sexual Abuse has examined the role of various authorities and organisations and their response to allegations of child sexual abuse.

It has looked at the way police respond to the abuse of children, but it has not examined police forces themselves.

A spokeswoman said the royal commission did not comment on its investigations, but factors that would affect whether an institution would fall under the commission's spotlight would include whether it had received a large number of accounts about a particular institution or group of institutions.

On Tuesday, Judge Carmody sentenced Bill to 19 years' jail, with a 15-year non-parole period, for his offences against the nine children between 1967 and 1983.

This will be combined with his sentence in NSW, of which he has eight years left to serve, for offences spanning 23 years.

Taking into account time already served, Bill now has a combined sentence of 26 years, with a non-parole period of 19 years. It is believed to be one of the state's longest for sexual predators.

When she stood up to defend her client in court last week, barrister Amelia Beech began by saying she could not argue with the gravity of the charges to which Bill was pleading guilty.

"There is very little that can be said."

Later, she spoke of Bill's feelings about his crimes.

"There is some evidence of remorse on [his] behalf," Ms Beech said. "I don't say that it's strong evidence of remorse ... [but] it seems, if I can say this, your honour, he is making the right noises."

But a forensic psychologist report described Bill's insight into his behaviour and the impact of his crimes on his victims as being at the "very formative level" and as "simplistic and underdeveloped".

He was assessed as being of "high risk" of reoffending, and Judge Carmody said this "reinforces the need for a lengthy term of imprisonment".

Bill won't be eligible for parole until he is at least 85.

Speaking about the effect of Bill's abuse, one of his victims told Judge Carmody she hoped Bill rotted in hell for what he had done to her.

She was just five when she was first raped, and the abuse continued for years.

"He is a monster," she said. "He needs to never see the light of day again ... once a monster always a monster – he will never change."

After the sentencing, she said she hoped Bill died in jail.

"I'm glad to see that justice is served, but I'm just relieved that it's all done," she said. "It's been a long time coming."

Bill will see out the term of his imprisonment at Ararat prison in Victoria, where he works in the number plates section.

When he was arrested, he cut off all remaining ties with his family. He receives no visitors.