A Christchurch man has today admitted stabbing his ex-wife to death in a horrific "frenzied" street attack before declaring, "We are going to die together."

Shiu Prasad, a 52-year-old diesel mechanic, married Keshni Naicker, 28, in 2013 but came 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, a court summary of facts says.

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

Prasad phoned and text 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.

She tried to stop a passing a car and get in a rear passenger door while Prasad ran after her.

Naicker took off towards the corner of Hanrahan St and Ilam Rd but fell over – 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 OK, 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.

Today at the High Court in Christchurch, Prasad pleaded guilty to murdering Naicker and wounding Sharan with intent to cause grievous bodily harm.

Justice Gerald Nation remanded him in custody for sentencing on December 7 at 2.15pm.

Detective Senior Sergeant Greg Cottam said police hope the guilty plea will go some way to helping the Naicker family, as well as Asveen Sharan and his family, to "move on with their lives" without having to go through a court trial.

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."