South Africa's power utility Eskom says it has shut down the second unit at its Koeberg nuclear power station for generator repairs that could run into weeks.

The repairs are likely to require Unit 2 to be shut down for a number of weeks. Unit 1 at Koeberg continues to operate at full power, Eskom said in a statement on Saturday.

Unit 2 was switched off on Friday night and Eskom does not expect the fall in supply to result in any shortage although it increases the risk of demand surpassing supply during peak hours, the state-owned company said.