The incident occurred at Senotlelo village in Mpumalanga minutes before midnight on New Year's Eve.
Colonel Leonard Hlathi, of the Mpumalanga provincial police, said yesterday that the 40-year-old man and his 26-year-old wife, of Mmametlhake district, were quarrelling at about 11.30pm while the five-year-old looked on.
The man apparently assaulted his wife before shooting her.
"The helpless boy rushed to his mother's parent's home and reported the incident. Family members accompanied him back to the scene and on the way there, they heard a gunshot.
"They arrived at the scene and found that it [the gunshot] was their son-in-law committing suicide. He died at the scene.
"The woman was then rushed to Mmametlhake Provincial Hospital in a serious condition.
"She died in hospital yesterday morning from the injuries she had sustained."
Police spokeswoman Captain Aafie Botma said towards the end of last month, a policeman was robbed of his uniform and firearm at his home in Ikageng, outside Potchefstroom. The suspect was found with two sets of police uniforms and a firearm belonging to the policeman.
Members of the public reported that there was an imposter seen several times wearing a police uniform.
On New Year's Eve, the "cop" was arrested.
Botma said the man would be charged with housebreaking and theft.
A 50-year-old warrant officer allegedly shot his wife in the face on Christmas Day at their home in Mmotong, near Polokwane.
The other officer, 46, allegedly shot his wife in the stomach and upper body on December 26.
At Mankweng, three men were arrested for allegedly stabbing to death a shop owner during a night-time robbery on December 21.