A leading national expert in domestic violence has given evidence at the trial of a woman charged with murdering her partner following years of abuse.

The Crown alleges Karen Anne Ruddelle murdered Joseph Michael Ngapera in November 18.

During an argument, Ruddelle picked up a 19cm kitchen knife and stabbed Ngapera twice in the chest.

One blow pierced his heart and he died almost immediately.

Advertisement

Ruddelle admits stabbing Ngapera but says the act was self defence and she was trying to protect herself and her teenage son, who had stepped between the feuding pair to intervene.

Her lawyers have mounted a defence of social entrapment - or battered women's syndrome.

Today is the sixth day of Ruddelle's trial in the High Court at Auckland before Justice Matthew Palmer.

The jury have heard from Rachel Smith, an independent family violence consultant who has worked extensively in New Zealand and the UK with victims and agencies.

Smith gave the jury an insight into intimate partner violence - and social entrapment.

She said it was crucial to understand and make visible all the forms of abuse in a victim's life - not just specific incidents.

"We need to comprehend her partner's entire pattern of coercive behaviours over time," Smith said.

"You cannot look at a victim's response in a certain moment ... She is responding not to an individual incident - she is responding to everything he has ever done to her.

Advertisement

"Victims experience their partner's abuse as cumulative - and they respond to it as cumulative.

"It is layers up on layers upon layers, it's like an onion and every layer counts and takes effect.

"What you need to understand is we might look at someone's action as a freeze frame but she's interpreting the threat she faces as all the cumulative things that have happened to her."

Smith said victims like Ruddell were "context experts".

"They know when situations are becoming dangerous," she said.

She explained domestic violence to the jury in detail, telling them it was not just about physical abuse but also included psychological and emotional harm, manipulation and control.

In a situation where a victim like Ruddelle snapped, the full context of the situation had to be considered.

"It is impossible to judge whether the victim's actions were reasonable without first understanding the nature of threat she was responding to and what options she had to respond to that threat," she said.

"That includes her partners' ongoing coercive control and how people have responded to a woman's safety needs over time."

Smith said racism, poverty and other forms of social inequity shut down a woman's safety options and compounded a person's ability to abuse them.

"We need to look at all of this to understand the victim," she said.

"Traditionally we've focused on the victim and what she has and has not done.

"Social entrapment ... is about shifting the focus to the person using violence, and their pattern of abuse over time - taking the point of view wider to the panoramic perspective and how people and systems have helped, or not helped.

"Did the actions they take make her safer or did they make her not safe?"

Smith also spoke about strangulation in her evidence.

Ruddelle told the jury on Friday that on one occasion Ngapera had "choked" her, held her against a wall with his hands tight around her neck.

"I thought I was going to die," she said.

Smith said non-fatal strangulation was a common form of control used by abusers.

"It lets (the victim) know that their partner is capable of killing them," she said.

"When you have your hands around your partner's neck, that is what you are communicating ... they strangle to show that they are capable of killing.

"You do not have to strangle your partner every day to show your ability to cause harm."

Smith addressed the "myth" that women were safer if they left abusive relationships or called police.

"We need to be really, really clear - separation does not guarantee safety," she said.

"They are still in an ongoing pattern of harm ... whether separated or not, they are not necessarily going to increase their safety.

"Continuing to report to police can cause them further danger."

She said there were a number of common misconceptions about victims.

"Just separate and you will become safer ... get a protection order, go into refuge ... call the police," she listed.

"People can often misunderstand the nature of the threat she is facing ... They don't look at all the things that were happening."

Smith said currently in New Zealand there were "very inadequate safety options" for women.

She also spoke about victims struggling to leave an abuser because they loved them.

"Victims of violence are often compassionate - they are just wanting the person who's harming them to experience a life where they are not harmed," she said.

"But because of coercion and control, what we often see is their partner may not have their wellbeing at heart - they can exploit that compassion and care."

Smith said women like Ruddelle were caught in a "catch-22".

"Without taking action they are likely to be harmed and by taking action they are likely to be harmed.

"It's a very dangerous situation to be in."

The trial continues.