The son of ''Black Widow'' Helen Milner has today been jailed for four-and-a-half years after causing a fatal car crash while driving with P in his system.

Adam Kearns, 26, was coming to the end of a home detention sentence on drug and gun charges when he crashed on a Christchurch road on December 9, 2015.

Police found that he was doing more than 111km/h in a 50km/h section of Wairakei Rd about 7.45pm when he lost control on a gentle left-hand bend.

The Subaru fishtailed, slid sideways, and smashed into a parked car, shunting it forward 21m, before coming to rest in the front yard of a house.

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His mate Nick Morrison, 24, died at the scene.

A female passenger spent five weeks in hospital with a fractured pelvis, fractured vertebrae, and broken wrist.

After the crash, Kearns returned a positive test result for the class A drug methamphetamine. The level of the drug has not been able to be determined.

He earlier admitted four charges, including dangerous driving causing death and injury to passengers, causing a death while driving with evidence of methamphetamine use in his system, and breach of home detention by consuming an illicit drug.

Today, Christchurch District Court heard emotional victim impact statements that outlined the devastating consequences of Kearns' actions.

The Morrison family has been denied seeing their "beautiful son grow to a full and complete man", mother Lisa Morrison said, through one "impulsive, stupid, reckless act by Adam Kearns".

"Not a day goes by, and seemingly not a moment passes, both day and night, that I don't think of Nick. What we had, what he was, and what he might have been," she said.

"We have to live with the loss of our very special son.

"No court could ever fix the situation, or give us our son back, but I have faith in our judicial system and its ability to deliver an outcome that reflects what Adam Kearns did and what we have lost.

"Adam Kearns needs to stand up and take responsibility for what he did to Nick."

His father, Brent Kevin Morrison, 56, owns and operates New Zealand's oldest jewellery business, which his son had just joined after enjoying his "OE" in Queenstown.

It was a "dream come true" for the proud father to have his caring, loving and talented son join the family business.

But after the shock death, Brent Morrison has gone from living and breathing his business every day, to having almost no interest at all.

"A great chunk of me is missing and I think of Nick every day," Morrison said.

Nick's sister, Freya Morrison, 26, said her brother liked helping people and that's what he had been trying to do with Kearns.

He helped Kearns get his home detention conditions altered so that he could go and live with him, to help get his life back on track and do something positive with his life.

Kearns' "careless and thoughtless actions" has made life "a lot darker than it used to be" and she questioned his remorse.

"Adam has caused so much harm in his life. He needs to understand this," she said.

The passenger, now 26, says she still suffers pain in her leg and back, which her surgeon says could stay with her for the rest of her life.

But she often thinks how lucky she was and wonders: "Why Nick and not me?"

Defence counsel Kerry Cook said Kearns accepted he had methamphetamine in his system, but that speed was the overwhelming factor in the fatal crash.

There was little he or Kearns could say to the Morrison family to assuage their feelings of loss but he said Kearns was clearly sorry and remorseful.

"You have killed a person and you have ruined lives because of your own sense of self-entitlement," Judge Tom Gilbert said.

"On any view of it, this was a horrific episode of driving. What was fortunate was that more people were not hurt."

He noted Kearns' criminal history, which included repeated driving offences, but he accepted that he was remorseful.

"Although it's rather too late for words, Mr Kearns, I accept you are sorry for what you have done and the harm you have caused," Judge Gilbert said.

Before Kearns was taken back into custody, the judge added: "You heard Nick's sister reading her victim impact statement, and at the end, she said this: 'You need to understand that you have to take responsibility for your actions and serve your punishment and then do what Nick would've wanted, for you to do something good in this world'.

"And that is perhaps the only way that you could, in a little way, make better what you have done. Give up the life you have lead to date and when you get out of prison, do something constructive with your life."

It is the latest episode in the troubled life of Kearns.

Helen Milner was jailed for the murder of her second husband, Phil Nisbet. Photo / File
Helen Milner was jailed for the murder of her second husband, Phil Nisbet. Photo / File

Kearns' testimony helped convict his mother of fatally poisoning her second husband Phil Nisbet in 2009. She is serving a sentence of life imprisonment with a minimum non-parole period of 17 years.

He later won $55,000 from his killer mum after she had framed him and put him behind bars. Milner had admitted perverting the course of justice and was jailed for two years and eight months when she sent herself death threats and claimed they were from her son.

Kearns was arrested over the threats and spent 18 days in custody, including his 19th birthday. It is alleged that police took 13 days to carry out a search warrant which would eventually clear his name.

He has also launched legal action against police for allegedly failing to properly investigate his case.

In July 2015, Kearns was sentenced at Invercargill District Court to four months' home detention and 250 hours of community work after being found guilty of punching and stomping on former All Black Justin Marshall in Queenstown.

The assault, described by Judge Michael Turner as "unprovoked, alcohol-fuelled street violence", came after Kearns and Marshall had been inside a Queenstown bar drinking in separate groups into the early hours of Good Friday, 2014.

Once they left, they had words on Shotover St, and Kearns told Marshall: "You think you're the f****** man, well, you're not."

A scuffle broke out and Kearns and his mate, Tai Samuel Neilson, 25, who last year was sentenced to five months' home detention for his role in the assault, started attacking Marshall.

Earlier in 2015, Kearns narrowly escaped a jail term after he was caught dealing cannabis and in possession of a gun.

He told police he had the cut-down single-barrelled sawn-off shotgun because his mother, behind bars for murder, had wanted him killed.

"My old lady is paying for a hit to be put on me," he said.