A policeman shot his wife and a man before killing himself in Krugersdorp, west of Johannesburg, on Friday, police said.

"At around 10 am, the officer and his crew member were doing a routine patrol when they stopped a vehicle near Millenium Primary School," said Captain Appel Ernst.

The officer alighted from the police vehicle, leaving his colleague inside.

He then opened the door of the vehicle he had stopped and shot two occupants, a male and a female, before shooting himself.

"The woman is believed to be the officer's common-law wife. The three of them died on the scene," Ernst said.

"The crew member is still in shock after witnessing the incident."

Police were investigating the motive behind the killing.