High-level designs for a $14 billion underground toll road linking Sydney's northern beaches to the inner west show that the project is well advanced, detailing the exact locations for exhaust smoke stacks and the alignment of three-lane tunnels.

A "Concept Design Report" marked Cabinet-in-confidence reveals the exact sites for ventilation buildings and stacks, substations and water tanks for the proposed $14 billion Western Harbour Tunnel and Beaches Link.

The latest leaked documents seen by Fairfax Media and the ABC highlight the extent to which detailed work on the project has already been undertaken. Despite this, the government maintains it is still too early to reveal aspects of the planned toll road such as the location of ventilation stacks.

Highly specific maps for the project show a smoke stack at Balgowlah in Sydney's north east near Serpentine Cresent and Bangaroo Street, surrounded by housing and near Seaforth Public School.