Transport Minister Andrew Constance said the $955 million cost of the work at Central,along with the rest of the metro line,was being paid for by the government's"asset recycling program"and the $7 billion from the sale of electricity assets after the last state election.

"This is going to untangle Central – Central is a rabbit warren. This is part of the first major step in this journey to see this entire precinct come to life,"he said.

The contract for the work at Sydney's busiest station comprises the excavation and construction of the underground platforms beneath platforms 13 and 14,lifts and escalators,and the 19-metre wide Central Walk concourse from Chalmers Street to the metro and suburban platforms.

Central Walk will open two years before the driverless,single-deck trains begin running on the second stage of the new metro line.

Mr Constance said there was"always going to be this challenge with construction disruption"but the government and contractors would seek to minimise the impact.

A bridge over rail lines to connect Regent Street to the rail yard has been completed,which will be used to access the construction site and for ongoing maintenance. Trucks will also use it to carry earth excavated from the site of the underground platforms.

The government is also expected to announce shortly the date for theseven-month closure from late this year of the Epping to Chatswood line,which will be converted to carry metro trains.

Mr Constance would not be drawn on whether the first stage of the metro line from Rouse Hill in Sydney's north west to Chatswood would be opened before the state election in March. The government has said only that it would be opened in the first half of 2019.

The government also has longer-term plans to redevelop the Central Station precinct.

More than 270,000 people use the station each day,and within two decades the number is forecast to rise to 450,000.