The Berejiklian government’s flagship metro rail project under Sydney Harbour and the central city will cost up to $16.8 billion to complete, more than $4 billion above what has been budgeted, a secret government document shows.

The rail line from Chatswood to the central city, and onto Sydenham in the south and Bankstown in the west, is critical to government plans to expand the rail network to handle surging commuter demand. The government originally budgeted the rail line to cost $11.5 billion to $12.5 billion.

But an internal review by Sydney Metro, the state’s operating agency delivering the railway, has forecast it will cost $16.8 billion to finish by 2024 due to blowouts across many parts of the City and Southwest project.

Transport Minister Andrew Constance, left, and Premier Gladys Berejiklian inspect one of the station sites last year. Credit:James Brickwood

The highly confidential budget review, completed about 18 months ago, shows the biggest projected cost overruns are for new trains and systems; excavating tunnels and sites for underground stations, such as at Martin Place and Barangaroo; and extensive work at Central and Sydenham stations.