Sydney lord mayor Clover Moore's demands for the Berejiklian government to ditch the final stage of WestConnex would cost the state billion of dollars, an inquiry has heard.

Major construction of twin 7.5-kilometre tunnels between Haberfield and St Peters to link the M4 and M5 motorways is about to begin as part of the $7.2 billion final stage of WestConnex, which will be followed by an underground interchange at Rozelle.

Sydney lord mayor Clover Moore has renewed calls for WestConnex to be halted. Credit:Michel O'Sullivan

Two weeks after taking over as the chief executive of WestConnex, Andrew Head said he believed the project had passed the point of no return following the awarding several months ago of the contract for construction of the tunnels between the M4 and M5 motorways.

Mr Head warned it would cost billions of dollars to unwind contracts and compensate multiple parties including contractors and the new owners.