A man who subjected his girlfriend's seven-year-old son to months of abuse as part of a religion-inspired discipline regime has been found guilty of murdering the boy.

The boy died at Kodi Maybir's unit in Oatley, in southern Sydney, in May 2013, about four months after the child and his mother, Kayla James, moved in.

An autopsy revealed the boy had trauma to the back and front of his head, causing subsequent brain swelling.

Maybir had pleaded not guilty to murdering the boy.

The court previously heard Maybir initially lied to police, telling them the boy was hurt when he fell off a pogo stick.

But during the trial, the New South Wales Supreme Court jury heard two other accounts of how the boy died, including that Maybir had been play-wrestling with the child.

The defence told the jury that Maybir and the victim enjoyed wrestling, and Maybir would throw the boy onto the couch. On this occasion, he said, there was an accident.

"He hit the floor hard. Mr Maybir didn't murder him," defence barrister Grant Brady SC said.

Prosecutors had rejected Maybir's offer to plead guilty to manslaughter and said the jury should convict him of murder.

The jury also found Maybir guilty of two counts of assault, producing child abuse material and an act of indecency to a sibling.

Detective Sergeant Trent Power said the boy's death had taken a toll on everyone involved with the case.

"Obviously we're very happy with the result today but it's still a very sad day," he said.

"The family of this little boy has to live on without ... the beautiful little boy that he was.

"I'd also like to thank the great team of detectives that worked tirelessly and with so much passion on this case.

"We've spoken to the family in Australia and New Zealand and they're obviously very happy with the result. An awful day for them as well despite the result they were looking for."

Detective Sergeant Power said the videos played during the trial were some of the worst he had ever witnessed.

Abuse administered under religion-inspired discipline regime

A few months before the boy died, James and her three young children moved into Maybir's unit, which was converted into a music studio.

During the trial, the court heard the victim had suffered months of abuse at the hands of Maybir leading up to his death, as part of a religion-inspired discipline regime.

The court heard evidence that Maybir, a Christian hip hop music producer, "held extreme beliefs concerning religion ... one which seemed to be eliminating the devil from people".

The court heard the boy's mother Kayla James went along with a bizarre system of controlling her family, introduced by Maybir. ( ABC News: Vincent de Gouw )

"Evidence will show the accused very substantially controlled [Kayla James] and introduced her to a bizarre system of controlling her family, which she went along with," crown prosecutor Christopher Maxwell QC said during his opening address.

The boy was repeatedly hit, yelled at and forced to run until exhausted, the jury was told.

The jury viewed home videos of Maybir physically assaulting the boy, including one in which Maybir encouraged the boy's siblings to hit their brother.

The court also heard accounts from witnesses who saw the boy being forced to run laps until he collapsed, at a campsite in Bulli, near Wollongong, just over a month before his death.

Witnesses also gave accounts of the boy being beaten, punched in the face and forced to sit outside in the cold in his soiled underwear.

Food and drink were also withheld from the boy as punishment for wetting and soiling himself, the court heard.

Mr Brady said Maybir introduced boot-camp style discipline to the family because James was becoming increasingly aggressive towards the children.

The jury heard Maybir's ideas were influenced by movies, television programs and attending such camps himself.

Maybir had pleaded guilty to a number of assaults, but his barrister said the child's mother was responsible for most of the injuries he had suffered.

James is already in jail serving at least 10 years after pleading guilty to the boy's manslaughter in June.

Maybir will return to court for sentencing on February 19.