Motorists on Australia’s busiest freeway face years of disruption and closures as part of the multibillion-dollar construction of another road tunnel under Sydney Harbour.

Parts of the Warringah Freeway, used by as many as 250,000 vehicles a day, will be temporarily shut overnight during construction of the Western Harbour Tunnel and subsequent upgrades to the adjoining motorway.

Substantial construction for the harbour crossing could begin in late 2021 and finish by 2026, according to an environmental report, which was released to the public on Wednesday morning.

The project includes twin 6.5 kilometre tunnels from an underground junction for the WestConnex toll road at Rozelle to the Warringah Freeway at North Sydney. The seven-kilometre Beaches Link will then extend from the Warringah Freeway to Balgowlah and Seaforth in the city's north.