The $4.3 billion M8 motorway between Beverly Hills and St Peters is open and transport authorities are expecting initial confusion for some motorists.
Traffic was allowed into the twin nine-kilometre tunnels from midnight on Saturday, with the trip taking about 10 minutes.
At the same time, a toll came into effect on the M5 East. The toll on both roads is $6.95 for cars and $20.86 for trucks.
Minister for Roads Andrew Constance said clear signs, including electronic messages and lane markings, are in place to give drivers plenty of notice when tolls come into effect.
"We're encouraging drivers to plan ahead and allow extra travel time, particularly around Marsh Street and the King Georges Road off ramps, which are the last exits before the M5 East and New M5 tunnel tolls for motorists," Mr Constance said.
"Follow the signs and make a decision early to be in the correct lane," he said.
"If you miss these signs and make a wrong turn, you should always continue driving until you find a safe location to stop and replan your trip. It is not safe to stop or reverse in a motorway or tunnel environment.
"As with any major transport infrastructure, it will take some time before drivers are accustomed to the changed conditions.
"We advise people to be mindful of safety and changes to road configurations.
The M8 is part of the massive WestConnex project, and the next stages will see the new tunnels linked to the M4, Sydney Gateway to the airport, and the the first stage of the M6 to Kogarah.
Premier Gladys Berejiklian said, "We have already seen the huge difference that the New M4 project has made for Western Sydney, and we expect similar time savings on the M8 tunnels.
"In the first few weeks we expect drivers to take their time to get used to the changes, before seeing the full benefit of the project."
Federal Minister for Population, Cities and Urban Infrastructure Alan Tudge said more than 18,000 workers were involved in the delivery of the project.
"WestConnex is being delivered in four major stages, including the New M4 (now open), the New M5 tunnels and the M4-M5 Link and Rozelle Interchange, opening in 2023. When complete, WestConnex will provide drivers with a 33km, traffic-light free network," Mr Tudge said.
The new St Peters Interchange, a remediated landfill site, includes links to Campbell and Euston roads and two new Alexandra Canal crossings, the Campbell Road and Gardeners Road bridges.
The government reminded motorists that those who spend an average of $25 a week on tolls over a year are entitled to free registration.
Drivers who spend an average of $15 a week over a year are entitled to half price registration.
