A jury has been told that a woman who fatally stabbed her partner after years of domestic abuse was suffering from battered women's syndrome.

And for the first time she has spoken - starting with growing up watching violence between her alcoholic parents and how at just 7 her mother would send her "up the road to the phone box to call police to come help".

Karen Anne Ruddelle has admitted killing Joseph Ngapera in November 2018 but denies it was murder.

She says she stabbed him in self defence after he came at her "like he always does" - and because she feared he was going to hurt her teenage son who tried to intervene.

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Ruddelle is on trial in the High Court at Auckland before Justice Matthew Palmer and a jury.

The Crown called evidence from 40 witnesses and read a statement Ruddelle's then 14-year-old son gave police soon after the stabbing.

He woke to his mother screaming for help and said he tried to push Ngapera away.

Then Ruddelle grabbed the knife.

Ngapera was wounded twice in the chest, the first blow piercing his ribs, lungs and heart.

The Crown also revealed that in the years leading to the fatal incident Ruddelle had called police 16 times about Ngapera - who she feared due to his violence, threats and intimidation.

This afternoon the defence opened its case and said Ruddelle was effectively suffering from battered women's syndrome - now known as social entrapment, where a man uses "coercive and controlling behaviours" to dominate his partner.

"To be clear from the outset the defence say to you Karen Ruddelle did not commit murder - she acted in self defence," said defence lawyer Paul Pati.

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"She acted as best as she could in the circumstances presented to her, especially with her son in danger

"She sought to protect her son - her youngest son, who came to her aid after she called out for help."

Pati said when she called for help she was trying to alert her adult son.

She did not "anticipate or foresee" that her teenage son would intervene physically.

"Yes she did grab a knife, yes she did stab him twice - you can't run away from that, we are not trying to run away from that," Pati said.

"But only in the belief she had no other option to her … or her son."

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Pati said the incident happened in "a matter of split seconds" in "circumstances Ruddelle did not anticipate".

He said Ruddelle's evidence would be lengthy but it was important to the jury.

"It is crucial you understand what she has going through from a young girl to a mother of children to the relationships she has had," he told the jury.

He said her experiences - especially in intimate partner relationships - had shaped her perceptions of what was normal and how to interpret her partners' actions.

"She has an unfortunate history of relationships with men where she has suffered violence, including with Joseph Ngapera, a man she loved and wanted to marry," Pati explained.

He said it was not the defence's purpose to "blacken" Ngapera's character or "make him out to be a horrifically bad man".

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"The truth matters - what happened in their relationship matters," he said.

"The importance of this is to assist you in terms of how … she sees the world, how she responds to violence and anger."

The defence will call evidence from Rachel Smith, a leading expert on intimate partner violence in New Zealand, who would explain the myths and misconceptions about the social issue and answer questions like "why didn't she just leave".

She would also explain social entrapment in detail.

The jury will also hear from psychologist Alison Towns, who specialises in intimate
partner violence, and who spoke at length with Ruddelle after Ngapera's death.

A number of others who know Ruddelle and could speak to her life and relationship with Ngapera would then be called.

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"As you have heard at the start of the Crown case, as you have just heard now … the defence say that Karen Ruddelle did not murder the man that she loved," said Pati.

"It is on the Crown to prove that she did … The defence need not prove anything but we want to tell you Ms Ruddelle's side of the story, she wants to tell her story."

Ruddelle then took the stand.

A number of her and Ngapera's family were in the public gallery to support her.

She started by speaking about her childhood in Mangere, where she was the oldest of six siblings.

Her parents were alcoholics and Ruddelle said from the age of 9 she effectively became a primary carer for her brothers and sisters.

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Violence was commonplace in the house and Ruddelle was "groomed" by her mother to run away and call police when things got bad.

She left college and got a job in a factory in Onehunga when she was about 15.

At 17 she was made redundant and at 19 the first of her seven children were born.

Ruddelle told the court her first serious relationship started after the birth of her second son when she was 23.

The man had recently been released from prison and she met him a at a family gathering.

The couple married in 1985, and split 11 years later.

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"Some of the years were very turbulent," she said.

"He abused me.

"He had an addiction - stealing, and I'd drink alcohol just to get through all of that."

Pati asked her to tell the jury more about the abuse.

"On one occasion he kicked me in my spine ... my mum made him angry, he took it out on me.

"I was on the ground and he kicked me continuously ... I hid in the wardrobe and he found me there.

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"He kicked me for a while, enough to put my spine out ... it hurt so much I just let him do it and it just went numb."

After that attack the man left.

The next day Ruddelle went to the hospital and to a chiropractor.

She said the police were notified and a molestation order - like a modern protection order - was put in place to prevent her husband from coming near her.

"He couldn't touch me again, he'd be arrested by the police if he did," she explained.

She went on to have five children with the man - but one died as a result of cot death.

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Ruddelle also suffered a miscarriage.

Her husband was also violent to the children, and her mother reported that to police.

"Nothing was done because I was afraid of (my husband's) retaliation - that he'd get angry," she said.

"He'd get angry if the police had to come around and charge him with assault."

She lived in fear of the man.

He was jailed for the attack which damaged her spine, and died in Rimutaka Prison of renal failure - soon after she was granted a dissolution of marriage.

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Her evidence continues and is expected to finish tomorrow.