Thousands of jellyfish have invaded Koeberg nuclear power station, causing a drastic reduction in electricity supply.

Eskom said Koeberg operations had been cut by 40 percent after an abnormally high number of jellyfish packed into the power plant's sea-water intake basin.

Spokesperson Carin de Villiers said although there were no safety implications for people, in the past few days thousands of jellyfish had died after getting stuck in the filters of the station's cooling condensers.

"They get sucked into the intakes and then trapped in the filter screens," she said.

"As a result of this, a high number of them are killed.

"What causes them in such unusual numbers we don't know. The station has continued to operate at about 60 percent throughout this period and will continue to do so."

She said the station was using sea water at the rate of 80 tons per second, enough to fill an Olympic-size pool in 30 seconds.

The station has already taken steps to reduce the number of jellyfish being killed and its emergency control centre has been staffed to monitor the situation.

"We have put up filter screens so we can get them off before they get caught up ... this gives them more chances of surviving," she said.

De Villiers said there was no electricity shortage in the province as extra power was being brought in from the Mpumalanga area via overhead transmission lines.

Leslie Rencontrie, director of electricity at the City of Cape Town, said the reduced supplyhad not had any effect on the city so far.

"It looks like the problem is reducing from their side and that should bring things to normal," he said.

Professor of zoology at the University of Cape Town, Charles Griffiths, said the appearance of large numbers of jellyfish was not unusual, although he could not give a reason.

"Trawler fishers have also experienced the same problem with fishing nets getting clogged up with jellyfish," he said.