Sydney Trains said the Southern Highlands line was the only one not experiencing delays.
"Customers are advised to delay their travel or seek alternative transport," the statement read.
Commuters on trains that were between stations are stuck until the situation is resolved, with some people takin to social media to complain the air conditioning had stopped working.
It's understood Sydney Trains will shortly begin restoring power to the network. The Transport Management Centre said the delays were expected to affect services throughout the day. Real-time public transport apps are showing some services running between 30 and 50 minutes late.