Faulty equipment inside an Ausgrid substation in south western Sydney is to blame for a massive power outage which closed both directions of the M5 East tunnel during peak hour, causing traffic chaos on Sydney streets.
The power failure in Canterbury happened just as thousands of commuters were trying to get home or travel into the city for Vivid, the AFL at the SCG and to ANZ Stadium for the NRL.
The power started to return about 6pm, with the M5 East tunnel westbound re-opening and the eastbound tunnel re-opening about 6.30pm.
However, it was a close call - with the tunnel minutes away from losing all battery back-up, which would have meant a reboot for the whole system, roads minister Melinda Pavey told the Seven Network.
"It's going to be a long time to clear the mess but we were also looking at maybe a six to eight-hour window of it being closed," she said.
With kilometres of traffic backed up across the city as a result of the outage, Ms Pavey advised motorists to "stay in the city as long as you can - delay your trip or at least catch a train home".
Up to 100 sets of traffic lights were also blacked out in Croydon, Canterbury, Bexley, Roselands, Kingsgrove, Wolli Creek, Mascot and Burwood Heights.
Light rail services were also affected, with no services running between Lilyfield and Dulwich Hill and a limited service running between Central and Lilyfield between about 4pm and 6.30pm.
Ausgrid emergency crews restored power to about 60,000 homes and businesses in Sydney's inner west and south west that were also affected.
Crews were on scene at the Canterbury substation investigating how the incident happened.