Major underground drilling has begun in Sydney's inner west on two metro railway tunnels that will run below the CBD and Sydney Harbour to form part of the Sydney Metro.

Premier Gladys Berejiklian and Transport Minister Andrew Constance today launched one of five tunnel boring machines — dubbed Nancy — at Marrickville.

The mega borers, which are the length of two Airbus A380s, will carve a total of 31 kilometres of tunnel from the inner west to Chatswood for the new Sydney Metro City and Southwest.

Ms Berejiklian said the project, which is due to be completed in 2024, would be a critical piece of Sydney's future.

"This project is a game-changer," she said.

"People scoffed at us when we said we will build a second harbour rail crossing and here it is.

"Today marks the start of the huge task of digging twin tunnels under the city, delivering Sydney's new world-class metro railway and building a stronger, better future for the people of New South Wales."

NSW Premier Gladys Berejiklian called the project a "game-changer". ( ABC News )

Two of the machines will drill from Marrickville to the new metro station sites at Waterloo, Central, Pitt Street, Martin Place and Barangaroo before they are removed.

Another two machines will dig from Chatswood to the edge of Sydney Harbour before a specially-designed machine takes over work on underwater twin tunnels.

Transport Minister Andrew Constance said all five machines would be running before the state election in March.

"We would expect to have the tunnelling complete hopefully within 18 months," he said.

The drilling machine will tunnel more than 8 kilometres from Marrickville. ( ABC News )

He said the borers have been specifically designed to cut through Sydney's geology and vibrations will not be noticed on street level.

"People will not have any sensation whatsoever," he said.

"Nancy is specially designed to cut through our city's unique sandstone and shale and will tunnel an average of 120 metres a week.

"These machines are underground factories — mechanical worms designed to dig and line the tunnels as they go so that Sydney Metro can be delivered as quickly as possible."