A severe power blackout brought chaos to downtown Sydney today, causing peak-hour traffic jams as street signals failed and leaving workers stranded in lifts as they tried to make their way home.

Failures at four high-voltage power cables virtually paralysed Australia's biggest city, with the Sydney Harbour tunnel partially closed to traffic and the Sydney Opera House forced to cancel all performances for the evening, local media said.

Office towers lost all power, paralysing some elevators, at around 4:45 pm local time (0545 GMT). Up to 70,000 homes and businesses, including some inner suburbs, were blacked out, said state broadcaster ABC, which was running on back-up power.

The main stock exchange had closed by the time of the blackout.

EnergyAustralia, which distributes power to Sydney city, said it could take several hours to restore power.

"There are four power cables that supply very large (electricity) sub-stations," an EnergyAustralia spokesman told local radio. "They have detected a problem and have automatically shut down for safety reasons."

The blackout comes two days after Sydney deliberately turned off its lights for Earth Hour 2009, when cities world-wide plunged into darkness for an hour to highlight climate change.