A Christian hip hop artist who systematically abused and then killed his girlfriend's seven-year-old son has been sentenced to at least 31 years and six months in jail.

Key points: Seven-year-old subjected to months of abuse

Seven-year-old subjected to months of abuse Ambulance called almost 24 hours after boy was assaulted and lost consciousness

Ambulance called almost 24 hours after boy was assaulted and lost consciousness Kodi Maybir and Kayla James initially said the boy had fallen off a pogo stick

By the time an ambulance was called to Kodi Maybir's music studio in Oatley, in Sydney's south, in 2013, Levai had already been dead for some time.

The call was made almost 24 hours after he was assaulted and fell unconscious.

Maybir and his girlfriend Kayla James initially said Levai had fallen from a pogo stick, but it later emerged he had been subjected to months of physical abuse.

A jury found Maybir guilty of murder in November.

Prosecutors had called for a life sentence.

Maybir performed under the name General Kopri and also produced music for others.

He introduced a cruel and degrading form of discipline to the family, which James went along with.

"The murder of Levai is an offence of great heinousness," Justice Robert Allan Hulme said.

"Levai was subjected to cruel, degrading and inhumane treatment at the hands of Mr Maybir.

"[But] I am not satisfied that it is at the extremity that would require the imposition of a life sentence."

Levai's grandfather, who cannot be named for legal reasons, said Maybir got what he deserved.

"But it doesn't bring back the grandson," the grandfather said after sentencing today.

"He won't be coming at the door again, or sitting at the table or going [to] parks with us, and that's the saddest part."

"What he done to a child is unbelievable."

The Homicide Squad's Detective Chief Inspector, Grant Taylor, said investigators were relieved by the sentence.

"This is a particularly nasty case because of all the video and photographic evidence which depicted the deterioration of young Levai right up until his death," he said.

"No homicide detective likes working on cases like this, but when you live and breathe an investigation such as this from the beginning to the end, it's very rewarding to see justice prevail."

Cause of death remains a mystery

An autopsy revealed the boy had suffered trauma to the back and front of his head, but exactly how he was killed remains a mystery.

Maybir gave a series of false accounts, including that Levai was injured while they were play-wrestling.

"He has provided outrageous and transparent lies to avoid responsibility," Justice Hulme said.

James is serving a 10-year minimum sentence for manslaughter and gave evidence against her former boyfriend at his trial.

She waited about 24 hours to call an ambulance after first seeing her son unconscious in Maybir's arms.

After they were told Levai was dead, Maybir told James, "He's in a better place now. He's with the angels."

"You didn't know Jesus until you met me. That was the only way I could help you," Maybir said at the time.

The court heard Levai was vulnerable to the abuse because of an intellectual disability.

Maybir's total, maximum sentence of 42 years took into account a series of earlier assaults and other offences, some of which were filmed.

He will be eligible for parole in March 2045.