The state government is running behind its own timeline on its much-vaunted new rail line between Parramatta and central Sydney, having failed to release the final business case amid concerns the cost might balloon above $20 billion.

Transport authorities had expected the final business case for the line to be completed by the end of 2018.

Sydney Metro West. Credit:NSW Government

The line is to run from Westmead through Parramatta and Olympic Park to the Bays Precinct at Rozelle and the central business district. The business case was to include confirmation of the number and location of stops.

But the government has not released the business case and, according to industry and Transport sources, the estimated cost of the line may have jumped to as high as $25 billion after land acquisition around station sites was factored in.