Keith Owen Goodbun, 62, checked his rifle to make sure the scope was "dead accurate" before he got into his four-wheel drive on the evening of October 6, 2016, and began the 250-kilometre journey to his estranged wife's home.

He had been married to Molly Goodbun, 59, for more than 40 years but their relationship had deteriorated to the point where an Apprehended Domestic Violence Order was in place forbidding him from going to the home unless invited.

Goodbun arrived at the house in Horseshoe Bend, a suburb of Maitland, just before 3am on October 7 after driving from his property near Taree and tried to force his way inside when his wife opened a sliding door.

The couple's older daughter Bionca Simmons, aged in her 30s, ran to get help and returned to see her father shoot her mother in the chest. She tried to wrestle the bolt action .22-calibre rifle away from him, letting off one shot in the process, and was hit in the head with the butt of the rifle.

Goodbun shot his wife another three times and threatened his daughter, who tried to give first aid: "get the f--- out of here or you're next".

Five neighbours heard the shots and called emergency services.

In a sentencing hearing at the Supreme Court in Sydney, relative Rosienne Johnson broke down and had to be comforted after reading out a victim impact statement from Rachel Goodbun, the couple's younger daughter.

Ms Goodbun said she and her sister had suffered significantly since their mother's murder, with her sister unable to cope with loud noises like slamming car doors.

She said her mother, a warm-hearted volunteer who cared about everyone, was "always willing to see the good" in her estranged husband but had finally decided to start a new phase in her life.

She said she wished her mother had the chance to see herself in a future where she was independent and happy.

"My mother did not just die, she did not just pass away peacefully, she was murdered," Ms Goodbun said.

"She died bleeding on the floor of the house that she spent so many years trying to make a loving, safe home.

"I don't know how much she was aware of in her last moments. That is still one of the things I struggle most about with her death.

"Did she understand the person she had spent a lifetime loving, wanting to see her dead? Did she see my sister fighting with my father, trying with everything she had to save her? I hope not."

Goodbun's barrister Richard Wilson said his client "effectively managed to end his world" and destroy his family when he committed the "totally unjustified and criminal act".

He said his client's judgment was affected by alcohol abuse, which had caused brain damage, and he had reached a low in terms of his mental state.

"He accepts that he deserves a severe punishment," Mr Wilson said.

In a letter to the court, Goodbun said he felt "deeply ashamed about being a cruel murderer".

Justice Helen Wilson said Goodbun's conduct appeared to be a "clearly thought out" and "rationally planned" decision to execute his wife.

She said his crime was committed "out of a sense of vengeance" because he considered his wife had taken everything from him and made him live in the bush.

"Almost any sentence this court will impose on him will most likely see him jailed for life," Justice Wilson said.

Goodbun will be sentenced on July 4.

The Sydney Morning Herald