HONG KONG (AFP) - An anaesthetist gassed his wife and daughter to death using a yoga ball filled with carbon monoxide, a Hong Kong court has heard.

Prosecutors told the High Court that Khaw Kimsun left the inflatable ball in the boot of a car where the gas leaked out and killed them, according to reports from court on Wednesday (Aug 22).

His wife and 16-year-old daughter were found on a roadside in a locked yellow Mini Cooper in 2015, in a case which initially baffled police.

The pair were certified dead at the same hospital where Khaw worked, and a post-mortem revealed that they had died from inhaling carbon monoxide.

Police found a deflated yoga ball in the back of the car.

Khaw has pleaded not guilty to two counts of murder.

Prosecutors said on Wednesday that Khaw, a 53-year-old Malaysian national, was having an affair with a student and his wife would not grant him a divorce.

They accused him of hatching a deliberate plot to murder his wife, the South China Morning Post reported.

Prosecutors said it was likely that Khaw had not intended to kill his daughter.

The court heard that in a police interview, Khaw had said he had urged his younger daughter to stay at home and finish her homework on the day of the deaths, according to Apple Daily.

Khaw had been seen filling two balls with carbon monoxide at Chinese University, where he was an associate professor, reports said.

He told colleagues he planned to use the gas on rabbits, but later told police that he had taken it to get rid of rats at home.

The trial continues on Thursday.