Separating construction of road tunnels for the third stage of WestConnex from an underground spaghetti junction at Rozelle in Sydney's inner west is designed to "de-risk the project" ahead of a sale to large investors, the City of Sydney claims, as it ramps up its opposition to Australia's largest motorway project.

The environmental assessment of the M4-M5 link, released last month, failed to commit to a final design or address community impacts and was based on unreliable traffic modelling, the council said in a submission guide outlining its objection to the third stage.

"Rather, it prepares the pathway for sale of Sydney Motorway Corporation to the private sector, removing from the responsibility, oversight and control of the government the final design, cost and implementation of the M4-M5 Link," it said.

The government is swinging into gear on the auction of a 51 per cent stake in Sydney Motorway Corporation, the state-owned entity set up to deliver, maintain and toll WestConnex, by giving potential bidders such as Transurban and large infrastructure investors until November 13 to lodge their interest.