Gauteng police spokesman Lieutenant-Colonel Tshisikhawe Ndou confirmed that a kidnap case had been opened at the weekend after Nomathemba Peach, a mother of three, vanished last Wednesday.
"Police are doing everything to find the woman and solve this matter," he said.
Peach's relatives said she was supposed to have gone to Klerksdorp to fetch her car and then visit her mother in Potchefstroom, North West.
"I found it strange that she did not arrive that day as agreed," said her mother, Zanyiwe Peach.
One of the missing woman's brothers, Vuyo Peach, who is an advocate, immediately advised his family to register a missing person's case at a local police station.
He said police had told him that the last signal on Nomathemba's cellphone was in an area between Klerksdorp and Potchefstroom. One of her pupils said they saw her get into a grey German sedan.
Nomathemba's colleague at Lukhanyo High School, Julian Bailey, said: "I spoke with her in the morning before 10am and she sounded fine.
"But when I called later to follow up on our arrangements she started speaking softly," he said.
"One thing I remember her saying was 'I'm in the middle of nowhere'." - firstname.lastname@example.org