The planned toll road linking Sydney and Wollongong has been costed at an extraordinary $18 billion – almost $12 billion more than the rail alternative that would cut the journey to about one hour.

The cost of the F6 Extension, which is also greater than the $16.8 billion estimate for WestConnex, is revealed in government documents seen by Fairfax Media and the ABC that raise doubts about the government's ability to build both projects at the same time.

Described as the "Gateway to the South" in the cabinet-in-confidence document seen by Fairfax, the 32-kilometre road would extend from a Westconnex junction at St Peters to President Avenue in Monterey via a nine-kilometre tunnel followed by a continuous tunnel along the Sandringham Peninsula to the Georges River at San Souci.

It would then travel above ground from Taren Point to Loftus and then through or adjacent to the Royal National Park to Waterfall, where it would connect with the Princes Highway.