South Africa power supply was under "extreme" pressure Monday and likely to remain so until end of the week after a technical fault at the country's sole nuclear plant, electricity utility Eskom said.

The main transformer of one of the two units at Koeberg power plant near Cape Town developed a fault late Sunday, stripping some 930 megawatts of electricity from an already vulnerable national grid, Eskom said.

The reactor was not affected by the fault, the utility said, while warning of possible rotating outages across the country for days to come.

"The system is extremely constrained today and for the rest of the week," Eskom said in a statement, warning "any extra load or faults in the system may necessitate the need to go into load shedding".

Eskom said the faulty unit had been fixed but would take time to get back to full capacity.

Koeberg's twin reactors contribute nearly 2,000MW to the national power grid.

Eskom has imposed rolling blackouts in Africa's most advanced economy in recent weeks as its ageing coal-fired plants struggle to provide the 30,000MW consumed each day.

The power shortages are regularly cited as one of the factors hampering South Africa's growth.

The central bank last week revised its 2015 growth forecast for Africa's second-biggest economy down from 2.5 percent to 2.2 percent -- specifically fingering the country's electricity woes.

South Africa has plans to build more nuclear power plants, but the proposal has sparked controversy due to the costs involved.