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

Jefferies, 46, 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.

He was back in court today 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 the Ranger - with clothes pulled up over her head, exposing her chest, and bra unclapsed - 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 bbq 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.

Kim Richmond.

In reading her victim impact statement to the court, Richmond's mother Raywynne told Jefferies "may you rot in prison as you left Kim to rot in the lake".

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 his actions were "deliberate and determined way 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, 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.

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 devestated the lives of of many including his children.