George Sean Warren appears for sentencing in the High Court at Greymouth.

A West Coast man has been sentenced to life imprisonment for fatally shooting a woman in front of her children after she alleged he indecently assaulted her.

George Sean Warren, 45, will serve at least 17 years before he is eligible for parole after he admitted murdering 43-year-old Hayley Jane Williams at her Greymouth home on October 22.

According to the summary of facts read by Justice Gerald Nation in the High Court at Greymouth on Friday, Warren allegedly indecently assaulted Williams in front of one of her children on September 30. He was charged by police on October 2.

"Warren was incensed at the charge and on several occasions ... made comments such as "I feel like blowing that b...... head off," he said.

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Warren was armed with a .308 rifle when a friend took him to Williams' house on October 22.

Seven children were inside, including Williams' three children, when Warren confronted her and demanded she get in the car.

Warren got angry when she refused and grabbed her by the hair, then struck her up to nine times around the head and face.

She cowered and told him she would drop the indecent assault charge, the judge said.

Warren pulled the trigger twice but the gun failed to fire. Williams told him there were children in the house, but he pulled the trigger again and shot her in the throat.

She died instantly.

One of Williams' sons fled to a neighbour's house when he saw Warren point the gun at his mother. Another hid in a wardrobe before finding his mother dead in their driveway.

Warren was arrested just north of Hokitika after a day-long manhunt.

He told police he shot Williams because she claimed he had indecently assault her.

"You know, at 45, put me away for three-and-a-half years for f...... something I didn't do. Oh well f... I rather go away for 10 for something I did," he said.

Williams' three sons were now in the care of their grandmother, Christine Mehrtens, who had a support person read out her victim impact statement at Friday's sentencing.

"I constantly think of the fear Hayley must have felt when the gun was pointed at her. It's so cruel.

"I found out about the death of my daughter from one of my grandsons calling me telling me she was shot at his home while he was there.

"I miss her so much ... I have visions of her walking out of her front door with a smile on her face and being happy. I don't see how I will ever get over this," she said.

She said her grandsons still had nightmares about being murdered.

"Hayley was so proud of her boys. I can still see her cuddling them and calling them honey. She won't see her boys at school prize giving or support them when they start high school. She was a great mum. I'm proud of her and how they had been nurtured by her."

Mehrtens told Warren she never wanted to see his face or hear his voice.

"You are nothing to me.

"What sort of person does this to a mum, or anyone, in her home where her sons were supposed to be safe," she said.

Crown prosecutor Claire Boshier said it was a "cold-blooded execution".

Williams was shot at point-blank range while children were inside.

"She was begging for her life. She told him she would drop the complaint and children were there. [Warren] shot her leaving her body for her children to find," Boshier said.

She said Warren had no remorse and no empathy for what he had done.

She did not offer any evidence for the indecent assault charge and it was dismissed.

Justice Nation said the murder could only be described as "inhuman".

West Coast area investigations manager Detective Senior Sergeant Jacqui Corner said Warren's sentence "reflects the gravity" of his offending.

"The small West Coast community was left shattered and baffled by this crime," she said in a statement issued after the hearing.

"I would like to thank the investigations team who worked tirelessly on this case, but there is still a lot of work to be done with two people still before the court in relation to this matter."

The other two men, whose names are suppressed, have been charged with concealing evidence of the murder.