“The marriage hadn’t been a happy one for some time before her death,” said Foong, whose children attended the same playgroup as the Malaysian family.

Foong said Wong once revealed her husband had a new girlfriend and identified the lover as Shara Lee, a tutor who taught Khaw’s children at their home between 2007 and 2008.

The prosecutors had earlier mentioned Lee as an assistant in a fake research project involving rabbits that Khaw set up to obtain the carbon monoxide.

Khaw’s colleague, associate professor Dick Chow Ho-ku, said Khaw told him he would bring Lee to the university to observe the first experiment for the project on May 13, 2015, nine days before Khaw’s wife and daughter died. She turned up towards the end of the experiment, Chow recalled.

Foong also described how the mental condition of Wong, whom she would go to yoga and play badminton with, had deteriorated so badly she needed counselling and medical help.

She said the situation improved towards Wong’s death in 2015 because she had grown to accept the situation.

Foong described Lily, who also died in the car, as the most energetic of the couple’s four children: Mei Mei, the eldest daughter, Carly, the third daughter, and Didi, the youngest, their only son.

While Carly and Didi sat in the public gallery and waved at their helper when it was her turn to give evidence, Mei Mei is expected to take the stand herself soon.

But she agreed in cross-examination the household kept two cats and she was once told they were there to catch rats.

The case continued before Justice Judianna Barnes Wai-ling on Monday.