Beullens, now 77, was sentenced on Wednesday after being convicted of six counts of indecent assault at the South Windsor-based institute and one count of buggery while on a boys' camping trip at Walgett in northern NSW.
Judge James Bennett jailed the former youth worker for a maximum of 13 years, describing the conduct as appalling, degrading, egregious and an abuse of his position of authority.
"The offences were committed against (the teenagers) in the course of his employment as a youth worker, trusted to take care of them," the judge said.
Daruk was a military-style institution for troubled youths which used strict discipline and punishments such as beatings and self-isolation to keep the teenagers, called inmates, in line.
For six to seven years while working at Daruk, Beullens engaged in various acts of sexual assault with the four teenagers, telling them that he could help make their lives easier if they complied.
He also warned them that if they told anyone what he had done, they would be punished for lying about a superior.
In handing down the sentence in Downing Centre District Court, the judge took into account the seriousness of the offences and Beullen's complete lack of remorse.
During the trial, Beullen denied the charges and accused the victims of either lying for monetary gain or mistaking him for somebody else.
A victim impact statement from one of the teenagers described the psychological and social impacts that the assault had for more than four decades.
"The court has thus in that document been given some insight into the magnitude of the impact of the offence," Judge Bennett said.
While a custodial sentence was required, the court also took into account Beullen's age and ill health, noting that time in jail would be more onerous for him than for other inmates.
The judge found there were special circumstances, shortening the non-parole period to eight years. Beullen will be eligible for parole on March 15, 2030.
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