Innocent bystanders who tried to save a woman from a frenzied street stabbing have spoken about their guilt and helplessness at being unable to save her life.

Three people stopped to try and help Keshni Naicker, 28, who was being attacked by her knife-wielding ex-husband in Christchurch on September 15 this year.

They saw Shiu Prasad, a 52-year-old diesel mechanic, chasing his petrified and bleeding former wife down Ilam Rd before she tripped and fell and he repeatedly plunged a large knife into her.

The horrifying scene continues to haunt the traumatised witnesses who today struggle to sleep at night and wish they could've done more to help.

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Prasad was today sentenced to life imprisonment with minimum non-parole period of 13 years at the High Court in Christchurch.

The court heard how he had married Naicker in 2013 but became enraged after she left him earlier this year.

The couple had been living in Christchurch but on September 8 had an argument, which led to Naicker moving out of the marital home.

The next day, the rest-home worker told Prasad it was over.

Unknown to Prasad, Naicker had met Asveen Sharan, a 31-year-old meat-process worker, online back in January.

After September 9, Prasad repeatedly asked Naicker if she was in a relationship with another man – which she denied.

Prasad phoned and texted her on multiple occasions, saying he still loved her, challenging her over having a new boyfriend, threatening suicide and saying it would be her fault, and threatening to expose her "perceived" infidelity to her family in Fiji.

On the afternoon of September 15, Prasad spotted his ex-wife and Sharan walking together near the Bush Inn shopping mall.

That night, Prasad grabbed a large kitchen knife with a 21cm blade and waited outside Naicker's work, waiting for her shift to finish at 7.45pm.

While Prasad waited and drank from a wine bottle, Sharan showed up to walk his partner home.

But as they started walking along Ilam Rd, Prasad ran up behind them and stabbed Naicker in the back. He then stabbed the new boyfriend in the head, inflicting a large wound that bled heavily.

Sharan fell to the ground and Prasad stabbed him another four times in the chest, back and shoulder while Naicker screamed at him to stop.

Sharan started fighting back and told Naicker to run for it.

A 29-year-old student was driving to the supermarket when she passed the scene, with her 39-year-old community support worker friend in the car.

Naicker was screaming and tried to get in the back of their car as Prasad chased her.

"He's got a f****** knife," the driver shouted in what she'd later call a "surreal" moment.

But the good Samaritans couldn't unlock the doors quick enough and Naicker ran towards the intersection with Hanrahan St before she tripped and fell - some 80m from where the attack began.

Prasad stabbed her in what was described as a "frenzied assault". Naicker received multiple defence wounds to her arms and hands as she fought for her life.

Despite his serious injuries, Sharan managed to walk over and see Prasad stab his girlfriend.

He tried to intervene, along with other shocked bystanders, but they were threatened by Prasad to keep away.

"It's okay, she's my wife," Prasad said.

"I'm going to die with her. We are going to die together!"

When he stopped his attack, he stood up and said, "I've got nothing left", and "I will kill myself".

He lifted his shirt and stabbed himself four times in the abdomen and threw the knife away.

Prasad then lay on his back beside Naicker and cradled her head.

Naicker, who received six "substantial" stab wounds, died in the ambulance at the scene while paramedics worked on her.

The pair who stopped to help today said they will never forget the sight of someone running down the road with a knife, chasing Naicker.

They remember wiping blood from her face as she lay there dying and feel guilty that they couldn't unlock the car and save her life.

In the middle of the night, they regularly phone each other "crying, often no words".

Prasad earlier pleaded guilty to murdering Naicker and wounding Sharan with intent to cause grievous bodily harm.

Sharan today spoke about losing his "perfect" partner who was the "love of my life".

They had talked about getting married and having children together, the court heard.

He told Prasad that he was "gutless" and had brought shame on himself.

"You have taken every dream away from me," Sharan said in a victim impact statement.

"Your anger and actions have caused so much pain. My life will never be the same. It was a nightmare and remains a nightmare."

Naicker's heartbroken mother, who lives in Fiji, says they used to talk and text every day. Naicker had shared her marital problems and said how Prasad was not interested in having a family.

She fears her grief will be life-long and is scared that Prasad will kill again once released from prison.

"I think about Keshni and cry," she said. "Our lives will never be the same."

Prasad is now a heartbroken and also broken man, defence counsel Andrew McCormick said.

He described Prasad as an ordinary man who's behaved in an extraordinarily bad way after his wife's decision to leave him.

Crown prosecutor Mark Zarifeh said the vicious and sustained killing showed a high level of brutality and callousness, motivated by obsession and possessiveness.

He highlighted the fact Prasad said in his police interview that if he couldn't have his wife, then nobody could.

The attack has had an immense impact, Zarifeh said, not just on Sharan and Naicker's family, but also on the witnesses to the graphic scene.

Justice Gerald Nation tried to reassure them today, to say they did the best they could in what must have been a frightening situation.

Detective Senior Sergeant Greg Cottam believes the tragic set of circumstances was "avoidable".

"It highlights the need for people to reach out for support in times of relationship breakdown and stress," he said.

"Police and our partner agencies are here to help – these include Women's Refuge, Shakti, Aviva, Te Whare Ora, He Waka Tapu, Stopping Violence Services, and It's Not Okay, to name a few.

"I hope that Keshni's life was not lost in vain, and that people who need it will reach out for help to prevent anyone else being hurt or killed as a result of family harm.

"This is particularly important in New Zealand's immigrant communities, where people may be unsure of the support that is available from both Government and non-Government organisations.

"These organisations can, and want to, help people in need, and can help you access other assistance if needed."

Cottam also acknowledged the brave actions of three young women and a man who went to the aid of the victims.

"What they saw was truly horrific and I would not wish that on anybody," he said.

"However, their actions were selfless, and for them to still help was truly courageous."