A former NSW Scout leader has wept as he was jailed for at least 21 years for molesting boys over more than two decades.

Mario Henry Aliverti, 61, had pleaded guilty to 12 child sexual assault offences, including one count of sexual intercourse with a person under 10, carried out between 1985 to 2007.

In the NSW District Court on Thursday, Judge Jane Culver jailed him for 31 years and six months with a non-parole period of 21 years.

The former truck driver appeared via video link from Long Bay prison while some of his victims also tuned in.

They were aged between seven and 15 at the time of the offending.

Aliverti had leadership roles in the late 1980s and early 1990s with a Scouts group in southwest Sydney and preyed on six of the boys in that time.

He later gained access to another three boys outside of the organisation between 1993 and 2007, with the victims reporting complaints to police from 2016 to 2018.

As the judge imposed the sentence, Aliverti had his head in one of his hands and occasionally dabbed at his eyes.

“There can be no criticism levelled at any victim delayed in reporting to police,” the judge said.

“He will serve his sentence as an older man … (but) each victim lives with the trauma every day and, for them, his offending is not stale.”

Aliverti was given a 10 per cent discount for his late guilty plea.

The judge noted Aliverti’s wife suffered from a medical condition, and was devastated and alarmed at the thought of losing him as a carer.

“Regrettably, sometimes these cases create victims of the family of the offender,” she said.

“That is what has happened here.”

With time already served, Aliverti will be eligible for release on parole in 2040.

-AAP