Hong Kong's top court on Tuesday quashed a scholar's murder conviction over the deaths of his wife and daughter, ordering a retrial.

Khaw Kim-sun, a former Chinese University medical professor, was found guilty in 2018 and sentenced to life in prison.

He was accused of putting a yoga ball filled with carbon monoxide in the boot of a car eight years ago, with the gas leaking out and killing the pair.

Five judges at the Court of Final Appeal ruled unanimously in favour of Khaw.

They agreed with his lawyers that the trial judge had misdirected the jury by telling them that a yoga ball stopper should be part of the evidence against him.

The ball in the car didn't have a stopper, but a year after the deaths police found one in a drawer belonging to Khaw.

"We have come to the conclusion that [the trial judge's] directions regarding the stopper could have steered the jury towards an impermissible line of reasoning in rejecting the possibility of [Khaw's daughter] using the carbon monoxide to kill insects," the top court's judges wrote.

They said that since there were other yoga balls in the house, the discovery of a spare stopper couldn't be a significant matter by itself, and it didn't have strong probative value in terms of proving that Khaw was the one who put the yoga ball in the car.

The judges therefore ordered a retrial with a new jury.