A "cold blooded" killer who executed a pleading Greymouth mother outside her home where seven children cowered inside has today been jailed for a minimum of 17 years.

George "Sean" Warren, 45, admitted murdering 43-year-old Hayley Jane Williams on October 22 last year, a month after she lodged a police complaint that he indecently assaulted her in front of one of her sons.

Warren was arrested and charged with indecent assault and released on court bail conditions that he did not contact Williams or go within 300m of her home.

In jailing Warren to life imprisonment with a minimum non-parole period of 17 years today, Justice Gerald Nation described Warren's crime as "inhumane and cowardly".

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The Crown said Warren's deliberate killing of a witness against him "strikes at the very heart of the justice system".

The High Court in Greymouth heard harrowing victim impact statements that described the victim of this "cruel and senseless murder" as a great mother to three sons who still cry for their mum.

The court heard how Warren walked up to Williams' Marsden Rd home on October 22 – where there were seven children inside the house, including her own three children - on the Sunday afternoon with a loaded .308 shotgun under his jacket.

Williams had alleged that Warren had indecently assaulted her earlier on September 28 last year and police had him charged.

One of her children told a specialist child interviewer that a "scary man" came to his house and was trying to touch his mum and was saying "really gross" stuff to her.

Warren would later say he saw it as a false complaint.

With his mind "fogged by the anger he was feeling", Warren aggressively confronted Williams on her porch on October 22 and demanded she get in his car.

When she refused, he became angry and grabbed her by the hair and hit her between five and nine times around the head and face.

She cowered and began pleading with him.

He presented the rifle at her and pulled the trigger but the gun failed to fire.

Williams moved to the driveway and again Warren pulled the trigger unsuccessfully.

"Sean, I've got the five kids inside," she pleaded.

The police say she was crouched low but Warren maintains that she was standing when he cocked the rifle again and this time a round entered the chamber.

He pulled the trigger a third time.

The pathologist concluded the firearm's muzzle was less than 1m away from her at the time.

The bullet passed through her outstretched left hand and into her throat and neck. It passed through the spine and upper chest cavity before exiting via the shoulder.

"The combined effects of the spinal injury, torrential blood loss from major arterial injury, and loss of airway caused her rapid death," the Crown summary of facts says.

One son saw Warren point a shotgun at his mother and heard her yelling at him to leave.

He then ran to a neighbour's house for help. He did not see the actual shooting.

"While at the neighbour's house, he heard a really loud bang," the summary says.

Another of her sons hid in a wardrobe "as per a prearranged safety plan" regarding Warren.

"Immediately after the shot was fired he came out of the wardrobe to locate his mother deceased in the driveway, and began to scream," the court heard.

"Another child present at the address, unrelated to the victim, was in the lounge. She saw the defendant arrive and point a gun at the victim. She heard a big bang but did not see the shooting. She was aged 8 years old."

A large-scale manhunt was launched and Warren was finally found walking on Old Christchurch Rd about 33km south of Greymouth and arrested by Armed Offenders Squad (AOS) officers.

"F****** oaths I took her out," he later told police.

"I'm on bail for indecent assault on her, that's why I f****** shot her. She brought her f****** own death sentence on herself, man."

But he then claimed he only intended to shoot her in the legs, "so I could see the street again".

"She yelled down the driveway. And I f****** pulled the trigger again (click) and then I f****** cocked it and seen the f****** bullet go into the f****** barrel, into the chamber and then she said her last words and then bang, that was it," Warren said.

It was a "cold blooded execution", Crown prosecutor Claire Bouchier said.

He "brooded" on the indecent assault charge for a month and told an associate he felt like killing Williams, the court heard.

Warren has a lengthy criminal history with convictions for assault, weapons, drugs and thefts dating back to 1990.

It was a crime driven by anger, defence counsel Phil Shamy said, after what he saw as a false complaint against him.

Warren, his lawyer said, denied that he had gone to kill a witness or complainant against him.

But Justice Nation concluded that Warren killed Williams because she made a complaint to police about him.

She had pleaded with Warren to spare her for the sake of children, and vowed to withdraw the indecent assault complaint. But Warren still went ahead and killed her.

After the sentencing, Detective Senior Sergeant Jacqui Corner said the sentence reflects the gravity of Warren's offending.

"The small West Coast community was left shattered and baffled by this crime," she said.

"Police's thoughts are with the 43-year-old woman's family at this time, and we are grateful for their support and co-operation throughout the investigation. They have requested privacy."