She remembers pushing herself between them, her father yelling the Arabic word for "slut" and her mother shouting "I didn't do anything, I didn't do anything".

She remembers her father pausing to look at her every so often before he continued to stab her mother.

Ms Haydar, then an 18-year-old university student, begged him to stop and felt a jolt of pain in her own body.

"I was trying to hold his hand back, the one that had the knife, and trying to push him off her but I couldn't," she said.

In the NSW Supreme Court on Wednesday, Ms Haydar gave evidence in the trial of her father who stabbed her mother to death in front of her in their Bexley townhouse in March 2015.

Haydar Haydar, 60, broke down into sobs as the youngest of his four children walked to the witness box to give evidence.

"I heard my mum scream ... I remember thinking 'I think he hit her' and I ran. [I saw] Mum backed up in a corner and he stabbed her in her back," Ms Haydar told the court.

"I tried to get in the middle of it, to pull him away from her ... He didn't stop, he kept going."

Mr Haydar has pleaded not guilty to murder but guilty to the manslaughter of his wife and mother of his children, Salwa Haydar, a 45-year-old drug and alcohol counsellor at St Vincents Hospital.

The defence case is that Mr Haydar warrants a reduction in the charge because was suffering a substantial impairment due to depression at the time of the killing.

Ms Haydar told the court that her parents had a tumultuous relationship, which worsened after her troubled brother hit her mother and was later jailed for drug matters.

The couple had discussed divorce multiple times, the court heard, and had each bought their own places but were living together in the Bexley townhouse owned by Mrs Haydar.

On the day Mrs Haydar was killed, Mr Haydar had returned unexpectedly and without telling his family from a trip to see his ill mother in Lebanon.

The court has heard that before leaving Mr Haydar, had discovered text messages sent from his wife to a male colleague and he had incorrectly suspected they were having an affair.

While overseas, Mr Haydar sent his wife a text message telling her she had made a "grave" mistake.

"You'll turn a grandmother soon and it's a pity that you beg for coffee from this and that, some of whom may take advantage of your weakness while you live in dreams. I say this at the time that my heart broken," the message said.

But Ms Haydar told the court she had not heard about her father's suspicions until the day he returned from Lebanon, when she noticed he was in a frantic and tense state.

He asked her and her sister whether their mother would ever be unfaithful to him and grabbed a Koran and swore on it that he had always been faithful to her.

Ms Haydar said that later that evening her parents had an argument that was triggered by her father's refusal to get the cooking oil from the garage.

When she heard a scream she ran downstairs to see her mother backed in a corner and her father stab her in the back.

Ms Haydar said she desperately tried to wrench the knife from her father, thinking that if she placed herself between them he would not hurt her.

In the process, she suffered two deep cuts to her hand. Mr Haydar is also facing two charges relating to those injuries.

"[I was] saying 'What are you doing? Oh my god, you're going to kill her," Ms Haydar said.

Ms Haydar said she begged him to stop stabbing her mother and when she picked up the telephone he turned and asked what she was doing.

"And he would like look back and continue," Ms Haydar said.

"I noticed him pause and then continue and then pause and look around."

Her father, she said, then picked up his briefcase and left the home.

The court has heard that Mrs Haydar was stabbed 30 times.

Mr Haydar later walked into a police station, with his hands covered in blood, and told the constable on duty: "My wife, I just stabbed my wife. I can't feel anything".

The trial continues in front of Justice Peter Garling.