Your smartphone knows plenty about you, and it's that health data that's apparently been key to solving an Australian murder case.

In September 2016, 57-year-old Myrna Nilsson was found dead in the laundry of her Adelaide home.

According to ABC News, her daughter-in-law Caroline Dela Rose Nilsson told police that a group of men had invaded the home and attacked her mother-in-law.

Bound and gagged, Caroline Nilsson said she didn't see the attack. But in March, police charged her with murder following an analysis of the victim's Apple Watch, with accusations the story was fabricated.

According to the report, prosecutor Carmen Matteo told Adelaide Magistrates Court the watch's activity and heart rate measurements indicated an attack occurred at 6:38 p.m. and that the wearer had "almost certainly" died by 6:45 p.m.

"If that evidence is accepted, it tends to contradict the accused's version of an argument occurring between the deceased and these men outside the laundry for a period of up to 20 minutes," Matteo said.

Caroline Nilsson told police she emerged from the house straight after the attack at 10 p.m. to call for help. If the Apple Watch data is accepted as evidence, Matteo said this would be more than three hours after the attack occurred. Magistrate Oliver Koehn refused bail based on the prosecution's case, and the matter will return to court in June.

It's another instance where fitness data has been crucial to criminal cases. Back in 2017, Connecticut man Richard Dabate was charged with murdering his wife, after Fitbit data on the victim showed inconsistencies with Dabate's account of the crime.

In a German case earlier this year, fitness data off an iPhone was used as evidence against a man accused of murder.