Warning: Story contains graphic content.

The Wellington man who fatally bludgeoned a woman to death so he could sexually assault her daughter is "evil", the woman's husband says.

Joseph William Borton was today given an indefinite prison sentence for the brutal attack, which dragged out for 18 hours and left the 12-year-old daughter needing skull reconstruction surgery.

READ MORE: Karori murderer Joseph Borton sentenced to life, preventive detention
Wellington man murders woman with mallet in Karori, sexually violates girl
Karori murderer Joseph William Borton seemed 'normal'

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The 31-year-old had earlier pleaded guilty to the attack, which happened in the affluent suburb of Karori on April 4.

He admitted attacking the girl and her 52-year-old mother with a steel mallet and sexually violating the child as her mother lay dying nearby.

Outside the High Court at Wellington this morning, Detective Constable Hayley Adams read a statement from the woman's husband.

In the statement, the husband said the death of his wife was "numbingly awful".

He described the victim as a "very special person" who was "outstanding" at her job, and a charismatic and compassionate member of her community.

"Her death was a great loss to society," he said.

His daughter is "miraculously" expected to recover well.

Support from police and medical professionals as well as the wider community helped them manage their grief.

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He said one stranger who had shown up to his house to pick up an item gave him a hug when she heard what had happened.

"A small act of compassion means a lot, and we have had so many kind acts."

"We live in a wonderful society and the crimes that were committed on my wife and daughter were the exception.

"The perpetrator is evil. It's not how we see our society."

Detective Senior Sergeant Warwick McKee said in his 30 years of policing and leading homicide investigations, this was the worst case he had dealt with.

"The offending committed here can only be described as abhorrent, cruel and utterly disgraceful behaviour."

He said Borton had been sentenced appropriately.

Justice Simon France gave Borton a life sentence with a minimum non-parole period of 19 years for the murder, but also imposed preventive detention for the sexual offending.

Preventive detention is an indeterminate sentence which allows Corrections to have control over the offender and, if released on parole, recall them to prison at any time.

Borton pleaded guilty in August to all charges, including murder, sexual violation, indecent assault, wounding with intent to cause grievous bodily harm, and several theft charges.

The offending was labelled by Crown prosecutor Grant Burston as being at a high level of "cruelty, brutality, depravity and callousness".

Borton was living in Lemnos Ave and had met the victims more than once.

On April 4, he saw them entering another home on the street and followed them into the house with a mallet.

The attack happened on Lemnos Ave in Karori. Photo / Melissa Nightingale
The attack happened on Lemnos Ave in Karori. Photo / Melissa Nightingale

The girl recognised him and said hello, but as she turned to catch a cat she was looking for, Borton swung the mallet at the back of her head, sending her flying, the summary of facts said.

"The defendant placed one hand around her throat and his other hand over her nose and mouth. She struggled for breath," it said.

The girl clawed at his hands, screamed, and tried to bite Borton's fingers, but was unable to get away from him.

He then struck her again in the head, this time caving her skull inwards and knocking her unconscious.

The mother heard the girl scream and came to help her, but as she entered the room Borton struck her repeatedly in the head with the mallet, fracturing her skull and causing heavy blood loss.

As she lay dying on the floor, he undressed the girl and sexually violated her, telling her "it's okay".

The girl then watched as he went over to the dying woman, who was groaning in pain. She fell unconscious again and Borton began undressing the woman.

Borton then returned to his own flat and showered, before coming back to clean up the blood from the victims' head injuries, the summary said.

When the girl regained consciousness again, Borton came into the room and told her there had been an earthquake, injuring her and her mother. He said her mother had gone to hospital and would be back soon.

He then undressed and lay down by the girl, saying "let's play". The girl lost consciousness again.

About 18 hours after the initial attack, Borton called 111 and said he had killed the woman, referring to himself as a "peeping Tom".

An autopsy revealed the woman suffered at least eight separate head injuries. They were so significant that fragments of her skull were pressed into her brain.

The 12-year-old needed surgery to reconstruct her skull and now has a plate in her head and has damaged sight and hearing.

Borton told police he intended to "incapacitate" the mother so he could sexually offend against the daughter.

The victims have name suppression to protect the child's identity.

Psychologist reports provided to the court offer "no ready explanation" for the attack, Justice France said. Borton has no previous convictions.

He said Borton had a dysfunctional personality, was growing increasingly isolated in the weeks leading up to the attack, and may have felt frustration and jealousy towards people who he perceived as having a better life than him. He was largely estranged from his own family.

All the report writers expressed concern he was downplaying the extent of his sexual fantasies.