Pell was jailed on Wednesday for sexually assaulting two 13-year-old boys after Sunday mass at Melbourne's St Patrick's Cathedral in 1996 and 1997.

The case rested on the testimony of only one victim, after the other died of a heroin overdose in 2014.

Those boys were close friends from primary school and sang together in the cathedral's esteemed choir.

But the deceased victim never told his parents of the abuse.

His father has paid tribute to the survivor, noting his bravery to raise the allegations also brought justice for his son.

"I commend the young fellow who has come forward," the father said.

"He kept it in for a long time and that would've been hell for him. Absolute hell.

"But I'm really appreciative and thankful that he did come forward.

"I want to give him a hug. He was a fabulous little kid. He's a fabulous man now."

But the father, whose name is withheld to protect his son's identity, believed Pell's sentence was too light.

Pell was jailed in the County Court for up to six years, with a non-parole period of three years and eight months. He denies any wrongdoing and has lodged an appeal.

"Our client is disappointed with the short sentencing and has expressed sadness over what he believes is inadequate for the crime," Shine Lawyers' Lisa Flynn said, acting for the boy's father.

The father is pursuing civil action against Pell or the Catholic Church over his son's death.

He believes his son began using heroin when he was 14 to block out memories of his abuse.

Pell's surviving victim appreciated Judge Peter Kidd's "considered" sentence but said the moment was "overshadowed" by the pending appeal.

"It is hard for me to allow myself to feel the gravity of this moment," he said in a statement, read outside court by his lawyer Vivian Waller.

"I appreciate that the court has acknowledged what was inflicted upon me as a child.

"However there is no rest for me. Everything is overshadowed by the forthcoming appeal."

The victim said it was a "difficult step" to tell police he had been abused by a "high-profile person".

"I have played my part as best I can," he wrote.

"Being a witness in a criminal case has not been easy. I'm doing my best to hold myself and my family together."

Australian Associated Press