Reaching over two of his teenage children, James Leonard Fairhall raised a knife and stabbed it into their mother's neck as she cowered in a corner of her son's bedroom.

Noeline Dalzell's children were protecting her from a man who had long been verbally abusive and intimidating.

Now, after their long-term on-off relationship had ended and she had moved on, Fairhall was angrier than his eldest daughter, just 16, had ever seen him.

Fairhall is charged with murdering Ms Dalzell in February last year while their three children screamed for him to stop.

Prosecutors have rejected his plea of guilty to manslaughter and he is standing trial for murder in Victoria's Supreme Court.

Days before killing Ms Dalzell, Fairhall learned she began a new relationship with an old school friend Kevin Blain.

On February 4 the three children - two girls aged 13 and 16 and a 15-year-old boy - arrived home from school to find their parents arguing, with Fairhall accusing her of cheating.

Their son yelled for him to leave the house. Their youngest daughter noticed Fairhall carrying a pair of scissors as he followed Ms Dalzell around the Seaford house. When she saw him put them down, she hid them.

Fairhall took hold of a large kitchen knife and made Ms Dalzell sit on a chair in the living room, while the older girl yelled at him to stop and let her mum go, prosecutor Jane Warren told Victoria's Supreme Court.

When she initially tried to get up Fairhall pushed his arm against her. She tried again and walked into a hallway, followed by Fairhall.

The two elder children pushed their mother into the son's bedroom to protect her, and backed into a corner, cowering when he came at the group.

With their younger daughter beside him, Fairhall ignored the screaming pleas of all three children to stop.

"(Fairhall) then reached over the top of (the son and daughter) with the knife ... and he then stabbed Noeline to the left side of her neck," Ms Warren said.

Almost immediately Fairhall was tackled by his son and Ms Dalzell was able to flee and ask a neighbour for help.

Pursued by Fairhall, he used a second knife to threaten the neighbour, telling him "don't call the cops".

He did anyway. Ms Dalzell collapsed before help arrived and could not be revived.

Fairhall's lawyer Jarrod Williams said that Ms Dalzell's violent death was undeniably sad and tragic and it was understandable jurors would feel sympathy for her and her family, and antipathy or anger toward Fairhall.

He said Fairhall admits killing her, but disputes that he intended to kill her and so is not guilty of murder.