Calling the idea “extremely dangerous”, Buckley went on to characterise the transporting of the deadly gas in such a fashion as “a crazy thing to do”.

The University of Sydney professor, who flew in from Australia, was called as an expert witness by the defence, who are looking to rebut the prosecution’s case that Khaw used a gas-filled yoga ball to kill his wife, Wong Siew Fing, 47, and daughter Lily Khaw Li Ling, 16, on May 22, 2015.

Prosecutors have said Khaw, who denies two counts of murder, placed a yoga ball leaking carbon monoxide in Wong’s yellow Mini Cooper on the day she and her teenage daughter were due to use it. The pair died of carbon monoxide poisoning, after they were found unresponsive in the car, which was parked at the Sai O Village bus stop in Ma On Shan.

Khaw has not denied filling up the ball with the gas, which the court heard he obtained from his university. But, the anaesthesiologist argued he only took it back to his three-storey village house to kill rats, prompting prosecutor Andrew Bruce SC to cross-examine Buckley about its safety.

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The Australian doctor said he would expect anyone with a relevant qualification to appreciate the danger involved in such attempt. As effective as it sounded, he said, it would be “unwise” to eradicate rats this way, as the gas was also dangerous to other species.

Khaw’s barrister, Gerard McCoy SC, used Buckley’s testimony to challenge the credentials of medical scientists the prosecution had previously called.

Prosecutors had put three doctors on the stand to establish that an experiment involving rabbits and carbon monoxide that Khaw conducted before the death of his wife and daughter was of “no clinical value”, and was simply a ruse to obtain the carbon monoxide.

But, on Friday, Buckley said the prosecution witnesses had relied on controversial research to reach their conclusions, and that one of them was not an expert in the field of carbon monoxide poisoning.

Earlier in the day, Khaw’s friend, Victor Cheong Mun Khan, a Malaysian plastic surgeon whom he met when they worked together at Chinese University, also testified on his behalf.

Calling Khaw a well-mannered, devoted family man, Cheong described his friend as a “very clever man who knows a lot, and he knows how to take care of things”.

However, Cheong said he discovered in 2007 that Khaw and his wife were no longer close, and said Wong had discovered Khaw was having an affair in 2013, from friends during a party at the couple’s holiday home.

He said Khaw later admitted cheating on his wife with his student, Shara Lee, who started out by teaching his children Chinese. The friendship later turned romantic, Khaw told him.

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During a post-arrest interview with police, Khaw told officers that Lily, who knew about the yoga ball and carbon monoxide, had used it to commit suicide.

But on the stand, Cheong remembered Lily as a “cheerful and happy” girl.

Eunice Li Lok-ting, the daughter’s ex-schoolmate, remembered her as a clever and rebellious girl, who excelled in humanities subjects.

She said when Lily, who enjoyed playing pranks on people, ran into trouble she was the type to ask others for advice before tackling something herself.

The trial continues before Mrs Justice Judianna Barnes Wai-ling.