A severe power blackout brought chaos to downtown Sydney during the evening rush-hour.

The power outage caused massive peak-hour traffic jams as traffic signals failed.

Elsewhere, many workers were stranded in lifts as they tried to make their way home.

Failures at four high-voltage power cables virtually paralysed the city, with the Sydney Harbour tunnel closed to traffic and the Sydney Opera House forced to cancel all performances for the evening.

Power was restored at about 6.40pm (8.40am Irish time).

Office towers lost all power, paralysing some lifts, at around 4.45pm (6.40am Irish time).

State-funded broadcaster ABC reported that up to 70,000 homes and businesses, including some inner suburbs, were blacked out.

Emergency services were also called to deal with dozens of people trapped in lifts.

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

EnergyAustralia, which distributes power to Sydney city, said four power cables that supply very large electricity sub-stations had detected a problem and had automatically shut down for safety reasons.

'We've been able to get power flowing back into three cables that supply the city,' a spokesman told AAP news agency.