Sydney motorists will have to wait longer than the Baird government planned for the second stage of the $16.8 billion WestConnex motorway to be opened to vehicles due to delays in environmental approval.

The scheduled completion date for the 11 kilometre-long New M5 has been extended until 2020, from 2019.

The delay means that the duplication of the existing M5 East between Beverly Hills and St Peters in the inner west will not be opened until after the next state election in 2019, depriving the state Coalition of something to showcase to voters.

Sydney Motorway Corporation, the body in charge of delivering the toll road project, said the completion date had been extended "as a consequence of the environmental approval being delayed for several weeks under the caretaker provisions of the federal election".