This was nothing new to Jessica Silva, who was used to his abuse and stayed calm during his phone calls on Mother's Day in 2012.

The pair exchanged more than 80 texts and calls that day, including this from Mr Polkinghorne: "F--- you and your pathetic thing of a mother. Hope she gets poison off the flowers you buy with my money and you will get gang raped."

In his final phone call to Ms Silva, Polkinghorne gave her 30 seconds to walk out the door "otherwise I'll be hitting f---in anger level hundred, straight off the bat".

That evening, Mr Polkinghorne was high on ice when he arrived at Ms Silva's family home in Marrickville, where he punched her face, tore her clothes, and scuffled with her brother and father on the street.

Ms Silva ran inside, got a knife and stabbed Mr Polkinghorne five or six times.

She was charged with murder, but in 2014 a jury found her guilty of the less serious charge of manslaughter.

In a majority decision, the NSW Court of Criminal Appeal on Wednesday quashed Ms Silva's conviction, finding she had acted in self-defence.

Justice Lucy McCallum said Ms Silva could only have felt Mr Polkinghorne's attack on her was "urgent, life-threatening and inescapable".

"Absent some act in self-defence, serious harm to Ms Silva, her brother or her father at the hands of the deceased must have seemed virtually certain."

Justice McCallum said a feature of the couple's exchanges was that Ms Silva seemed to have normalised Mr Polkinghorne's "vile abuse and the most terrifying threats".

"In many calls, she listens quietly as he rages on, abusing her and blaming her for all that was wrong with the world.

"The circumstances described in the evidence in this case are the kind in which, more commonly, it is the woman who is killed."

Acting Justice Robert Shallcross Hulme proposed the acquittal, finding that once Ms Silva saw Mr Polkinghorne straddling her brother she must have been certain the police would not arrive in time.

"It was also not obvious that the combined efforts of those present could overpower the deceased."

Acting Justice Hulme noted that Ms Silva stabbed Mr Polkinghorne repeatedly, and in the back.

"However the deceased's verbal and physical conduct and strength so reeked of violence that the requirements of reasonableness did not demand that [Ms Silva] avoid doing what she did."

Justice Mark Leeming was the dissenting judge, arguing that it was open for the jury to have found Ms Silva's actions were unreasonable.

Ms Silva threw her head back in relief as the judgment was delivered in court.

Outside court, her lawyer Adam Houda​ said: "Today's decision sees that history corrected and preserves her legacy, which is extremely important for my client."