A doctor in Hong Kong is on trial accused of murdering his wife and daughter using a yoga ball filled with carbon monoxide gas.

Prosecutors said the man, a Malaysian anaesthetist named Khaw Kim-sun, was having an affair with a student at the hospital where he worked, and that he had become estranged from his wife and four children.

Mr Khaw’s wife, Wong Siew-fung, was found dead alongside their 16-year-old daughter in their locked Mini Cooper stopped at the side of the road, with a deflated yoga ball in the boot.

The case was at first a complete mystery to police, until a post-mortem revealed they had died from breathing a very high quantity of carbon monoxide.

Mr Khaw was accused at the start of the trial in Hong Kong of putting the leaking, gas-filled yoga ball in the car on the day his wife and daughter died, 22 May 2015, according to the South China Morning Post. He was arrested in September.

Mr Khaw had reportedly told colleagues at the teaching hospital that he wanted to "test the purity” of the carbon monoxide and its effects on rabbits.

Later he told police he had wanted to use it to exterminate rats at home, although a domestic helper employed by the family testified that they had no rodent problem.

Prosecutor Andrew Bruce said Mr Khaw was unaware his daughter was home from school on the day of her death and likely had not intended to kill her.

“The last thing the accused wanted was for his 16-year-old to die,” Mr Bruce was quoted as saying.