NSW Premier Gladys Berejiklian on Wednesday launched the mega boring machine "Nancy" at Marrickville. The machine will munch through more than eight kilometres of sandstone and shale to the CBD.

Ms Berejiklian said it was the beginning of what would be a "game-changer" for transport across Sydney.

"I remember very vividly in 2012 when we launched our 20-year vision for transport, people scoffed at us when we said we would build a second harbour rail crossing and here it is," the premier told reporters.

Transport Minister Andrew Constance compared the project - which will eventually involve five borer machines - with London's Tube and the Paris Metro.

"These machines are underground factories, mechanical worms designed to dig and line the tunnels as they go," he said in a statement before telling reporters "people (above ground) will not have any sensation whatsoever".

Nancy is named after Australian pioneer aviator Nancy Bird Walton.

"She'd be passionate about this because it's something that could change the future of Sydney," Ms Walton's daughter, Anne Marie Holman, told AAP on Wednesday.

Nancy and another tunnel boring machine will dig 8.1 kilometres from Marrickville to the new Sydney Metro station sites at Waterloo, Central, Pitt Street, Martin Place and Barangaroo.

Two TBMs will also dig 6.2km from Chatswood to the edge of Sydney Harbour. A fifth machine has been specially designed to deliver the twin tunnels under the harbour.

The five machines will excavate 5.9 million tonnes of rock the equivalent to fill 940 Olympic swimming pools.

The entire $20 billion-plus Sydney Metro rail project is scheduled to be completed in 2024.

Australian Associated Press