City Power Johannesburg says a high voltage Eskom power line feeding most of its substations in the north of Johannesburg has tripped.
This has caused a "major power outage" in the northern areas of the city, including Midrand.
"We apologise for the inconvenience, our technicians are attending to it," it said in a tweet just before noon.
City Power did not say what caused the power line to trip.
Power utility Eskom, meanwhile, instituted stage 4 load shedding on Monday due to a shortage of available capacity.