The cost of a second road tunnel underneath Sydney Harbour is estimated to be $7.2 billion, 9NEWS can reveal.

A leaked Transport for NSW document shows the project’s "Estimated Total Cost" is $7.2 billion, with a proposal for an 80:20 funding model with the Federal Government.

The document lists a request of $5.76 billion from Canberra to help pay for the project.

The tunnel will link WestConnex at Rozelle, cross under Sydney Harbour between Birchgrove and Waverton, then connect to the Warringah Freeway at North Sydney and the Northern Beaches Tunnel.

The Western Harbour Tunnel will link WestConnex with the Warringah Freeway (9NEWS)

The estimated cost of the second under-harbour crossing is $7.2b (9NEWS)

Opposition Leader Luke Foley claims the request for federal funding shows the government is struggling to pay for their new projects.

"People deserve to hear an honest account from The Premier. What projects they will deliver, in what order and how they will be paid for," Mr Foley said.

Government sources say plans requesting federal assistance for the Western Harbour Tunnel were preliminary and they have since decided not to seek any for this project.

However, they may need help on rail projects like the Western Metro.

"We will be going to the community when we have done our homework on exactly how much those projects will cost and how we are funding them," Premier Gladys Berejiklian said.

The location of the Western Harbour Tunnel

How the tunnel will link with other major road infrastructure

Ms Berejiklian was also non-committal on whether the smoke stacks along many of the motorways being constructed will have pollution filtered.

The Government claims they are still waiting on advice from the independent Air Quality Advisory Panel.