The final part of the WestConnex motorway project linking the M4 and M5 motorways will be widened from three to four lanes in each direction, and exit and entry ramps at Camperdown in Sydney's inner west ditched, under new design plans.

The Baird government says the designs for stage three of the controversial $16.8 billion project could also allow the new twin tunnels for the M4-M5 Link to be opened in 2022, a year earlier than planned.

Under the new plans for the toll road to be unveiled on Thursday, the tunnel alignment has also been shifted west to make a more direct route between interchanges connecting to the M4 at Rozelle and the M5 at St Peters.

The government said the changed alignment of the tunnels meant Royal Prince Alfred Hospital at Camperdown and its Cyclotron medical facility, and the University of Sydney, would not be impacted.