Cory Jefferies will spend at least 11 years in jail for the murder of his partner of 26 years, Kim Richmond.

The 46-year-old was today sentenced in the High Court at Hamilton to life imprisonment with a minimum non-parole period of 11 years for the murder in July 2016.

Jefferies denied intentionally killing Richmond with his lawyer Tom Sutcliffe telling a jury in the High Court at Hamilton in July that it was an "unscripted event" that saw him panic.

But they didn't believe him, finding him guilty of murder after six and a-half-hours' deliberations.

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He was back in court for sentencing by Justice Sally Fitzgerald this afternoon when he was jailed.

"They have lost their mother and in many ways they have lost their father as well," she told Jefferies.

She said although she accepted there were financial pressures at the time, she said it was also likely due to stress from him discovering she was having an affair with her neighbour, Alfons Te Brake.

"You perhaps saw Miss Richmond slipping away from you after 26 years."

Justice Fitzgerald described the way Jefferies left his partner in the back seat of their Ford Ranger - with her clothes pulled up over her head, exposing her chest, and bra unclasped - as "degrading".

The breach of trust, the attempt to cover up his crime and level of callousness in the treatment of her body after her death were all aggravating features, she said.

Richmond went missing after the couple attended a function at the rural Arohena Hall where they watched the rugby and had a barbecue until the early hours of July 31, 2016.

They then helped a local resident clean up before leaving the venue about 3.30am.

It was on that journey home that her death occurred after GPS from their cellphones stopped about 120m short of their home.

A violent attack took place either in or around in their vehicle.

The Fitbit recorded a period of elevated heartbeat between 3.30am and 3.39am as the couple helped tidy the hall before they set off and it calmed down.

Her last recorded heartbeat was at 3.43am.

Cellphone records helped detectives track Jefferies' phone as it travelled to Lake Arapuni before it headed back home.

Richmond's cellphone GPS signal remain where the vehicle had first stopped.

GPS was able to track Jefferies as he slowly made his way home.

Emotion in the packed public gallery ran high as Raywynne Richmond read her victim impact statement.

"May you rot in prison as you left Kim to rot in the lake," she said to Jefferies.

He was arrested the day before her funeral, when he had been laughing and joking about which songs to play.

She said they had kept some of Kim's clothes, a jersey of which was worn to school by her daughter.

"She asked if she could wear a jersey to school .. 'nana smell it, you can still smell mummy'."

Richmond's two sisters Tina and Tracey sobbed as their mother spoke of the devastating impact Kim Richmond's death has had on their family including her children.

Also in the gallery were Jefferies' neighbours Heather and Alfons Te Brake, who listened intently to the sentencing.

The pair were witnesses in the trial after Jefferies pointed the blame at Alfons Te Brake for an affair between him and Kim that Raywynne dismissed as never having existed.

When Jefferies' lawyer Thomas Sutcliffe said his client was "miserable" with regret over the murder, Richmond's family and friends scoffed.

Crown prosecutor Jacinda Foster said Jefferies' actions on the night he killed Richmond must have been "explosive and violent".

His actions over the following 11 months, as police and members of the public searched for her body, were "cold-hearted and manipulative", she said.

She said he was "deliberate and determined" to purposefully mislead her family and friends "for the purpose of covering up his involvement in her death".

Leaving her at the bottom of the lake robbed Richmond of her dignity, she told the judge, and showed a "profound" level of callousness.

Jefferies still refused to divulge how and why he killed his partner which she submitted "was a self serving way to minimise his culpability".

She said he only pleaded guilty to her manslaughter after being faced with "overwhelming evidence of her killing and subsequent disposal of her body".

"The steps that he took were well considered, they were careful and ultimately they were largely successful but for the perseverance of police."

Foster categorised the murder as "amongst the most serious murders that have come before the court and the level of callousness is particularly high".

A pathologist said Richmond's body had deteriorated so much a definitive cause of death was unable to be recorded, however he said it was likely due to either a head wound or neck compression.

Justice Fitzgerald did not accept Jefferies' last minute explanation that he killed Richmond with a closed fist, back-handed blow to the head in the ute.

And neither did Raywynne Richmond when asked outside court.

Defence counsel Thomas Sutcliffe said the fact it took so long for his client to be uncovered as the killer showed how respected he was in the community, he said.

"No one who knew him ever thought he would be capable of doing this ... and to a certain degree that is why this offending went undetected for so long."

Sutcliffe said Jefferies maintained his position that her death was "unscripted" and accepted that he had devastated the lives of many including his children.

Detective Inspector Graham Pitkethley, Waikato police crime investigations manager, said the sentence reflected the gravity of Jeffries' offending.

He described their investigation as not only thorough but also "challenging and extensive" of which his team worked tirelessly.

"Staff worked hard to conduct many extensive interviews with a large number of witnesses which were crucial to the outcome today."

He thanked those involved in the inquiry for their co-operation and support.

"The family of Kim Richmond was left shattered by his crime and our thoughts are with Kim's family at this time."