Details for a fitness tracker device owned by a woman allegedly murdered by her partner has revealed the moment her heart-beat stopped being recorded.

Cory Jefferies, 46, is accused of murdering 42-year-old Kim Richmond and is on trial in the Hamilton High Court.

Ms Richmond went missing from her South Waikato home in July 2016 and her body was discovered in nearby Lake Arapuni 10 months later.

The Crown said today the Fitbit device was never found, but details from it were sent to the cloud.

It said the explanation for the device stopping was either that it was faulty, not attached or that Ms Richmond's heart had stopped.

Mobile phone tracking also showed Ms Richmond's phone being at the lake's boat ramp, but not moving.

The Crown has claimed Mr Jefferies killed Ms Richmond in a seven-minute period on a drive home from a late night at a local function.

It said no cause of death was found due to the body being in the water for so long.

The Crown said Ms Richmond was in a relationship with another man and Mr Jefferies told the man he wanted her dead and to be rid of her.

The Defence said Mr Jefferies was responsible for her death, but was guilty of manslaughter, not murder.