Driverless trains carrying passengers under Sydney Harbour are expected to have an impact similar to the city's first crossing almost a century ago.
Dark concrete tunnels where driverless trains whisk passengers under the harbour provide little by way of scenery, but the modern metro also offers continuous mobile coverage for passengers who prefer to look at their phones.
Up to 250,000 commuters per day are expected to use the long-awaited line after it officially opened on Monday.
Sydney's iconic harbour has always posed challenges navigating the city but almost a century on from the Harbour Bridge opening, the city's latest crossing is expected to have a similar transformative effect.
The cross-city extension runs from Tallawong via Chatswood in the city's north to Sydenham in the inner west via tunnels under Sydney Harbour, forming part of a metro network that is the largest public transport project in Australia.
About 1000 people set their alarms early to step onto the first service from Sydenham at 4.34am, while a 4.38am train from the north was also packed.
Services continue until almost midnight before starting up again on Tuesday morning.
The $21 billion line's opening brings the biggest change for commuters since the city centre first linked with its northern suburbs in 1932, Transport Minister Jo Haylen said.
"I'm pretty sure that before the 'coat hanger' was in place, no one would have conceived that infrastructure development," she told reporters.
"Now, discovering a fast, automated train under our harbour, the vision of the former government and the vision of all of those people involved is now being played out across our city."
The project links previously hard-to-reach parts of Sydney, said Premier Chris Minns while paying tribute to his Liberal predecessors for making the metro line happen while they were in government.
"Gladys Berejiklian deserves a lot of credit for this city-shaping piece of infrastructure ... she had the vision to get it done," he said of the former premier and transport minister.
He also urged commuters to be patient as the new line integrates with the rest of the public transport system.
Opposition leader Mark Speakman said the previous coalition government delivered the first metro line and guaranteed the latest extension, but the most important thing was that it had finally opened.
"We have reached this great milestone, which will see a massive uplift in the transport capacity of Sydney," he told reporters.
He said the transformative project would never have been built under Labor, which cancelled long-running plans to construct it in 2010.
The NSW Liberals took out billboards and branded placards at several metro stops highlighting the line's design, funding and construction during their tenure.
The publicly owned, privately operated metro line will enable passengers to travel from Sydenham to Chatswood in 22 minutes, and from the city centre to North Sydney in three minutes, with turn-up-and-go services.
Some 445 new metro services across eight stations will run each weekday on trains that carry more than 1150 people and hit speeds of up to 100km/h.
The line's opening was pushed back 15 days after it became clear the rail safety regulator would not give approval before an earlier targeted opening date of August 4.
The ongoing Sydney Metro project includes four metro lines, 46 stations and 113km of new rail.
