A 16-year-old high on synthetic cannabis who kicked and bashed an elderly man dying of cancer during a robbery, before dumping him in a Christchurch carpark, has today been jailed for three-and-a-half years.

Moses Eli Hurrell's troubled upbringing was outlined during his manslaughter sentencing at the High Court in Christchurch for the fatal attack on Pierclaudio Raviola.

Raviola, a 65-year-old bus driver and chef, was attacked when he showed up at a Bromley house on March 23 last year.

Hurrell, now aged 18, punched and kicked Raviola during the attack, the court heard.

In what was described as a "callous and cynical" act, an unconscious Raviola was bundled into the boot of a car and dumped at a surf life-saving club carpark in the seaside suburb of Sumner.

Hurrell then used keys found on Raviola to burgle his Phillipstown home.

Raviola was found the next morning. He was rushed to hospital in critical condition with a fractured skill and swollen brain but died two days later.

This morning, the court heard of Hurrell's chaotic upbringing where drug, alcohol, violence and crime were constants.

He was prenatally exposed to alcohol and solvents and adopted when he was aged two. A pre-sentence report notes whānau affiliations across several gangs.

His first drinking happened when he was just seven years old, the court heard. He'd smoked synthetic cannabis on the night of the fatal assault.

Justice Nicholas Davidson noted Hurrell had been exposed to alcohol and substance abuse through "most of his young life", which, he said, explained his long criminal history in the Youth Court and why he was in court now for manslaughter.

Defence counsel Tony Garrett referenced Hurrell's "extremely challenging" birth and growing up with a "cultural lack of attachment".

The Crown, Hurrell's lawyer and Justice Davidson all agreed that drug and alcohol treatment was paramount in the young offender's rehabilitation prospects.

Justice Davidson was concerned that he'd seen "scant evidence" that the available rehabilitation has a proven track record.

Hurrell admitted anger issues and that he had thought it "cool" to use drugs and break the law.

But he has made progress during the past 18 months in custody, the court heard. He has good social skills, plays guitar and piano, and previously excelled at rugby and swimming, the court heard.

A remorseful Hurrell wrote to Raviola's family in Italy to apologise and say he takes full responsibility for his actions.

"At the time I was not thinking straight," he said.

"I know Mr Raviola was a good man to you and you all must miss him."

Hurrell said he is learning to be a better person and that when he gets out of jail, he hopes to find his real family.

"You have talents not all people have, indeed few have, and now you have to build on them," said Justice Davidson, who believes that Hurrell "stands at the "crossroads" in his life.

"For your own good, your whānau, iwi, Mr Raviola's family, and society, you should follow the path which will lead you away from crime. This is the moment of truth for you."