Justice Mathew Downs noted there were several aggravating features about the offending, including that it spanned 40 years - albeit with a hiatus after 1997.
"You were, of course, in prison for some of this time."
Much, if not all, of the offending involved a breach of trust, he said.
"You cultivated relationships with the victims to facilitate the offending. It was thus premeditated and cynical."
The most recent crimes were against a boy in the neighbourhood who on one occasion fled through a window to escape.
His mother told the court today the boy suffered nightmares so bad he would wake up in the middle of the night crying out like he was in "excruciating pain".
Crown prosecutor Tiffany Cooper argued for preventive detention to be imposed.
"The real concern that the Crown has is the fact that Mr Morris still does not accept his offending."
Reports before the court showed some "windows" where he was prepared to accept some of the offending but that was not something he sustained, she said.
"There is a real concern about the ongoing safety of the community."
The Court of Appeal gave him a warning in 1997, when a preventive detention sentence he was serving for indecent assault was quashed – a "final" warning, she said.
"We still find ourselves here in 2020."
Defence lawyer Jonathan Hudson argued a long finite prison term, with the possibility of an extended supervision order upon release, would be adequate to protect the public.
Hudson also noted Morris would be subject to the Child Sex Offender Register.
Justice Downs said he believed the Court of Appeal would not have quashed the 1997 preventive detention sentence - replacing it with a six-year prison term - had it known about all of the offending to that point.
He concluded preventive detention was necessary as no other sentence would be adequate.
Most of the offending was historical, but not all, he said.
"You have been committing sexual offences against children and young people for much of your adult life," he said.
"You are at best ambivalent about what you have done. You describe shame but you did not plead guilty."
In giving the sentence, the Judge set a minimum non-parole period of five years.
Today, the court heard how Morris' offending had affected the victims.
One statement, read by the police officer in charge of the case, spoke of how difficult it had been coming forward and confiding in others what had happened.
He simply had not been able to tell his mother and to this day she still had no idea, he said.
"It would crucify her."
He was living with awful memories "that will never go away" but hoped by testifying he could lay some of those demons to rest.
He wanted this "horrible man to pay" for what he did.
The offending had turned the victim into a hypervigilant and nervous man, prone to worry.
During the trial, he had to sit there and tell strangers "what this monster" had done.
After giving evidence he broke down but was relieved he got through it.
Another statement, read by the same officer, spoke of how the victim felt like his life could have gone down a totally different path.
The actions of Morris had made him "feel boxed in with nowhere to go".
It had contributed to his involvement in crime, the court heard.
"I still suffer the consequences of his actions to this day."
