A former Chinese University professor is appealing his conviction of murdering his wife and daughter with a yoga ball containing carbon monoxide, arguing it was one of the women who pulled the ball's plug and caused the gas leakage.

Khaw Kim-sun, a 55-year-old former associate professor at the university's department of anesthesia and intensive care, was sentenced to life in prison two years ago for murdering his wife, Wong Siew-fing, 47, and daughter, Lily Khaw Li-ling, 16, on May 22, 2015.

The two died of carbon monoxide poisoning inside their car when the gas leaked from the ball.

Khaw, who has been in jail for two years, appeared before high court chief judge Jeremy Poon Shiu-chor, vice president of the court of appeal Andrew Macrae and judge Kevin Zervos yesterday.

Defense counsel Franco Kuan Bak-on said the yoga ball could have been taken to the car and had its plug removed shortly before the incident, as the two victims died of high levels of carbon monoxide poisoning.

Kuan reminded the court that Khaw had left home an hour before the incident to attend a two-hour ceremony at CUHK. Therefore, it was "very likely" that either his wife or daughter put the yoga ball in the car and pulled the ball's plug.

According to records from the car key, Khaw had not opened the car door on the day the incident happened, Kuan added.

Kuan said the car left their home at 2.18pm and was found parked at a bus station at 2.25pm, showing the car's concentration of carbon monoxide must have been very high to make Wong and the daughter unconscious so quickly.

He said if Khaw had put the yoga ball in the car an hour before that, most of the gas would have leaked out before the duo got in the car.

Kuan also cited an expert report saying if the door was open for 30 seconds, 90 percent of the carbon monoxide would leak out of the car.

But judge Zervos pointed out that there was no evidence to show the original concentration of carbon monoxide in the car before the door was opened and therefore the calculation cannot be considered actual evidence.

Although their domestic helper said the two did not bring the ball to the car, Kuan said it's possible the helper failed to notice that because she also said they did not carry handbags or umbrellas - items later found in the car.

The hearing continues today.

Khaw Kim-sun was convicted of murdering his wife, above, and daughter, left, who died in a car from gas poisoning.

Prof appeals family murder verdict