Kim Richmond's Fitbit recorded her last heartbeat at 3.43am on July 31, 2016.

Her partner of 26 years, Cory Scott Jefferies, admits killing her, but says it was unintentional, or manslaughter and not murder.

Jefferies' week-long trial began in the High Court at Hamilton today before a jury of five men and seven women.

The Crown alleges that her death was carried out by her jealous partner after he became aware she was having an affair.

Cory Scott Jefferies in the dock in the High Court at Hamilton yesterday. Photo/Alan Gibson.

He discovered the affair seven months prior to her death and repeatedly approached the married man, Alfons Te Brake, and each time he attempted to calm an angry Jefferies.

Crown prosecutor Ross Douch said Jefferies told him "I just want to f****** kill the bitch" and that he "wanted her gone".

In the weeks before her death, Jefferies obtained photos of the pair kissing and again confronted Te Brake.

Richmond, the mother of Jefferies' three children, went missing during the early hours of July 31, 2016, after the couple had been at a function at the Arohena Hall where locals had gathered to watch a Chiefs semifinal game.

When Kim Richmond took up triathlons, Corey Jefferies joined her and they raced together at multisport events.

Her body was eventually found 11 months later in Lake Arapuni - where police had searched earlier - inside her Ford Ranger.

Families of both the accused and the victim's family were back in court to hear opening submissions from Douch, who said the couple had enjoyed a good night up until the event finished up about 3am.

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

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

It is then the Crown alleges that a violent attack took place 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 did not make the trip and stayed where the vehicle had stopped.

However, the tracking was also able to tell that it was moving at a much slower pace - a walking pace.

Jefferies eventually reported her missing on the Monday when he rang Richmond's mother, Raywynne, asking if she had heard from or seen her daughter.

Cory Scott Jefferies arriving at the High Court at Hamilton yesterday. Photo/Alan Gibson

Defence counsel Tom Sutcliffe told the jury his client accepted causing her death in the vehicle that night but it "was culpable homicide in that he committed manslaughter and not murder".

"Cory Jefferies admits that he is guilty of manslaughter but did he actually mean to kill her or mean to cause her bodily injury that will cause her death. What was the intention when something happened in that car?" he asked.

Eleven witnesses are due to be called in the trial, including Richmond's mother, Te Brake and other community members.