Jessica Silva, 24, has pleaded not guilty to murdering James Polkinghorne, 28, outside her family's Marrickville home on Mother's Day, 2012.

On Thursday Ms Silva broke down when graphic photographs, taken during a post-mortem examination of Mr Polkinghorne's body, were displayed on screen in the NSW Supreme Court.

Seeing her distress, Justice Cliff Hoeben who had already given Ms Silva dispensation to sit with her solicitor rather than in the dock, allowed her to leave the courtroom.

Forensic pathologist Matthew Orde said Mr Polkinghorne had suffered one stab wound to the back of his head and four stab wounds to the back of his torso.

Dr Orde said Mr Polkinghorne had suffered a "devastating injury" which was "almost inevitably fatal".

The case will test how far a victim of domestic violence can be pushed before she can kill in self defence.

Ms Silva's four-year relationship with Mr Polkinghorne was characterised by endless abuse. He threatened to kill her many times, often punched her in the face, head-butted her, wished gang rape upon her and turned up to her parents' house in an ice-fuelled rage on Mother's Day, 2012, threatening to "cave her head in".

But two hours before Mr Polkinghorne was fatally stabbed outside the Marrickville home, Ms Silva, 24, was recorded in a phone call to her brother saying that her on-again-off-again partner was a "f---ing psycho" and she was going to "f---ing stab him myself".

The young mother is on trial for murder in the NSW Supreme Court in a case that will test how far a victim of domestic violence can be pushed before she can kill in self-defence.

In his opening address on Wednesday, Crown prosecutor Eric Balodis told the 12-member jury that there would be little contention about the years of confronting, shocking abuse.

Nor would there be contention that Mr Polkinghorne sold and took ice, or crystal meth, and was paranoid and enraged on the night of May 13, 2013, when he arrived at the Silva family home.

"The Crown accepts that to strike a woman once is one time too many," Mr Balodis said.

"However, and there is a very considerable 'but' that I need to make plain ... there are lawful ways to respond to domestic violence."

Ms Silva had broken up with Mr Polkinghorne and moved out of their Dulwich Hill apartment two months prior.

In the days before his death, he was staying in a city hotel and using ice. He repeatedly tried to get back with Ms Silva, which she refused.

On Mother's Day, he called and texted Ms Silva dozens of times, calling her a "f---ing slut", saying she deserved to be raped and threatening to come over to smash up her parents' house and "cave your head in", the court heard.

The calls and texts were being recorded by the police as part of a separate investigation into Mr Polkinghorne, who was the main suspect in the murder of a drug rival, Nikolas Argiropoulos, whose body was dumped in a Birchgrove park in March.

"I'm just gonna bash the f--- out of you and I don't give a f--- who's standing next to you. I will bash them too," he told Ms Silva.

She tried to placate him and even apologised for not being more helpful in the relationship, defence barrister Greg Scragg said.

At 7.23pm, she called her brother Miguel, saying Mr Polkinghorne was being a "f---ing psycho" and fearing he was coming over to kill her. Miguel told her to call the police and keep the door shut if he turned up.

"I'm f---ing sick of it," she told her brother. "I'm not scared of him. I will f---ing stab him myself."

Mr Polkinghorne made his last call from a taxi bound for Marrickville just after 9pm, the court heard.

"Thirty seconds to walk out that door, otherwise I'm hitting anger level 100, straight off the bat," he said.

When he arrived, Miguel and his girlfriend were outside the house. Miguel is expected to give evidence that Mr Polkinghorne ran across the road and started swinging.

When Ms Silva ran outside screaming, he punched her in the head twice and ripped her pants, the court was told.

She ran inside, grabbed a kitchen knife and came outside to see Mr Polkinghorne on top of her brother, choking him.

She allegedly stabbed him four or five times to the back, shoulder and head and he died on the road while Miguel and Ms Silva's father restrained him.

Mr Balodis told the jury that the accused's response "was not lawful".

"She had other alternatives to killing [him] that she well knew," he said.

However, Mr Scragg said that there was "no doubt that he was coming there to either kill her or assault her or cave her head in … and if anyone got in his way, police or family members, whoever, it wasn't going to stop him."

"All she ever wanted was the deceased to stop hurting her and her family," he said.

The trial continues.

- with Rachel Olding