Mirna El-Aziz told a NSW Supreme Court jury on Wednesday she confronted the stranger as he screamed for her son at the fence of her Greenacre home on November 13, 2015.

She said the man she now knows as John pushed her to the ground before he "pushed" a kitchen knife into the bare chest of her son, who was seated on the porch.

John Can Yavuz, 37, has pleaded not guilty to murdering 22-year-old Charlie El-Aziz on November 13, 2015.

"He lunged on him," she told the jury before making a stabbing motion with her right arm.

She said her son had earlier come into the home with bruises and scratches before she heard his eventual attacker screaming "something like 'come and face me like a man'".

But despite warning the screaming stranger she'd call the police if he didn't leave, she said he walked over her small front fence, shoved her away and approached Mr El-Aziz .

Under cross-examination, she was asked whether she remembered Yavuz ask Mr El-Aziz "Why did you do that?"

"It all happened so quickly," she said.

"I didn't think of many things... I couldn't believe this was happening. I was sort of in shock."

She agreed with Mr Yavuz's barrister that at the time of his death, her son was on bail for an aggravated robbery charge with a condition that he live at her home.

An apprentice plumber who'd been working next door later told the jury that he heard a shirtless man at the front fence yelling words to the effect of "come out, get on your knees and apologise".

Youness Elmbrat said that before the stabbing, the two men had been arguing and punching each other across the street before one ran off.

"He said 'I'm going to come back and I'm going to kill you'," Mr Elmbrat, now aged 20, told the jury.

Australian Associated Press