By Sam McKeith in Sydney

The launch of Sydney’s $21 billion metro line will be just as transformational for the nation’s most populous city as the Harbour Bridge’s opening nearly a century ago, the NSW government says.

Up to 250,000 commuters are expected to use the long-awaited driverless train line under the city centre and harbour each weekday after its official unveiling on Monday.

The cross-city extension, which runs from Chatswood in Sydney’s north to Sydenham in the inner west via tunnels under Sydney Harbour, forms part of a metro network that is the largest public transport project in Australia.

Transport Minister Jo Haylen says the line will be the biggest change to how the city moves since the opening of the Sydney Harbour Bridge, which linked the business centre with northern suburbs in 1932.

“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.”

Premier Chris Minns said the project would link previously hard-to-traverse parts of Sydney, paying tribute to his Liberal predecessors for making the metro line happen while they were in government.

“The previous premier, Gladys Berejiklian, deserves a lot of credit for this city-shaping piece of infrastructure … she had the vision to get it done,” he said.

He also urged commuters using the link to be patient as the line became properly integrated with the rest of the public-transport system.

Liberal leader Mark Speakman said the previous coalition government was responsible for delivering the first metro line, from Chatswood to the northwest, and guaranteeing latest extension.

He said the transformative project would never have been built under Labor, which cancelled long-running plans to construct it in 2010 during the tenure of then-premier Kristina Keneally.

The NSW Liberals took out billboards and branded placards at several metro stops highlighting that the line was designed, funded and built during their tenure.

The publicly owned, privately operated city 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 on timetable-free, turn-up-and-go services.