The first underground stage of the nation's biggest road infrastructure project, WestConnex, is expected to open in Sydney this Saturday and the motorists' association is calling for a toll-free period.

Key points: The twin tunnels are three lanes wide in each direction and connect Homebush to Haberfield

The twin tunnels are three lanes wide in each direction and connect Homebush to Haberfield Drivers will pay a maximum of $4.27 to travel on the new section

Drivers will pay a maximum of $4.27 to travel on the new section The entire WestConnex project is due to be finished in 2023

Known as the M4 East, the twin tunnels will link to the widened M4 and connect Homebush to Haberfield in the inner-west and cost drivers $4.27 per trip.

The $16.8 billion WestConnex project is the centrepiece of the Berejiklian Government's $90 billion infrastructure program.

The 5.5 kilometre-long tunnels are three lanes wide in each direction and expected to take around 10,000 trucks off Parramatta Road.

The new M4 will have a maximum-distance based toll of $7.89 and the maximum toll for the section between Homebush to Haberfield will be $4.27.

The NRMA is calling for a toll-free period as soon as the new motorway opens.

"It will provide the public with an opportunity to experience the project, to drive on the motorway, and then make an informed choice about whether or not they want to use the project or whether they want to use a free alternative," head of media Peter Khoury said.

The M4 tunnels will allow motorists to bypass 42 sets of traffic lights. ( Supplied: NSW Government )

Premier Gladys Berejiklian said the newly opened section would shave 20 minutes off a peak hour journey from Parramatta to the CBD.

"We appreciate the importance of giving people back time," Ms Berejiklian said.

The Government considered a toll-free period but decided it against it, she said.

"On this occasion we thought it would be detrimental to do that, simply because of concerns about backed up traffic," she said.

"We didn't want to see kilometres-long queues in the new tunnels."

Ms Berejiklian has previously said that within two years, around 67,000 trips are expected in the M4 tunnels every day.

Eventually, WestConnex will link the M4 Motorway to the M5 Motorway, CBD and airport via 33km of tunnels and motorways.

When the project is complete, the NSW Government says motorists will be able to bypass 52 sets of traffic lights between Beverly Hills and Parramatta.

The second stage of WestConnex has already begun, with construction underway on the new M5 — 9km of twin tunnels that will connect the existing M5 at Kingsgrove to St Peters in the inner-west.

That section is expected to open in 2020.

The final stage of WestConnex is the proposed M4-M5 link which will create a non-stop bypass underneath the western edge of the Sydney CBD and inner-west.

Stage three will also include links to the Iron Cove Bridge and the Rozelle interchange.

The entire project is slated for completion in 2023.

"Once all of WestConnex is complete in 2023, drivers will be able to get from Penrith to the Anzac Bridge without having to stop at a single traffic light and travel times from Parramatta to Sydney Airport will fall by up to 40 minutes," Ms Berejiklian said.

NSW Premier Gladys Berejiklian and Transport Minister Andrew Constance are expected to announce the opening of the tunnels later today.