The security footage, broadcast by the eNCA television channel, shows the man striking the woman to the ground on a petrol station forecourt before launching a sustained assault.
Two uniformed officers are also filmed at the scene, making modest, and ultimately unsuccessful, efforts to restrain their colleague.
But the solidly built man strikes again and chases the woman, kicking her. As she falls to the ground he stamps on her head.
The two uniformed officers allow the man to leave the scene without arrest. The assault on the woman was filmed at a gas station on April 5 shortly after 9pm.
The man was identified as a warrant officer in Smithfield, a small town in the Free State province. He was in civilian clothes and arrived at the station in a sedan car.
The incident is being probed by the country's police watchdog. "We are investigating it," Moses Dlamini, spokesman for the Independent Police Investigative Directorate told AFP.
It is the latest incident of South African police brutality captured on film.
In February explosive footage emerged of a Mozambican migrant taxi driver being dragged down a road while handcuffed behind a police van. He later died in police custody.
Last year, police shot dead 34 striking platinum mine workers, with some of the deaths caught on camera. – AFP