A massive blackout has knocked out power to 50,000 homes and businesses in Sydney's north, creating traffic chaos and causing delays to the city's public transport system.
Energy Australia has denied the blackout is because electricity infrastructure is in poor condition.
A problem with the TransGrid network, which owns and manages electricity infrastructure and supplies Energy Australia, caused the problem about 5am (AEST) on Wednesday.
At least 60 sets of traffic lights are also not working.
Affected areas include Pennant Hills, Berowra and Hornsby.
"Just after five there was a problem on the TransGrid network which is the upstream network which supplies Energy Australia's network," an Energy Australia spokeswoman told Macquarie Network.
"That automatically shut down power to that part of our network which supplies those areas."
She said it remains unclear what the problem with the TransGrid network was or when it will be fixed.
A company spokesperson said engineers arrived at the substation shortly after 6am (AEST) and hope to identify the exact nature shortly.
Asked if the network was dilapidated and in need of repair, the spokeswoman replied: "I don't think that's the case. I know a heck of a lot of money's been pumped in to the electricity networks, certainly in Energy Australia's network and I know we're spending another $8 billion over the next five years."
She said it was unusual for a problem to occur on the TransGrid network.
Transport NSW says delays on the North Shore, Northern and Newcastle lines have been cleared, as back-up power kicked in.
However, it warned there is only limited lighting at some railway stations between Cheltenham and Hawkesbury River and urged passengers to take extra care boarding and getting off trains.
Hornsby station's lighting is unaffected, it added.
Some ticketing machines also may not be working, Transport NSW added.
It said passengers may be required to buy tickets at their destination.
