About 40 metres below the surface at Concord in Sydney's inner west, a giant cavern more than five metres high and almost 12 metres wide gives a sense of the scale of the $16.8 billion WestConnex project.

Some 21 boring machines have been working around the clock for more than a year to carve out about 1 million cubic metres of sandstone, much of which has ended up at housing development sites in the city's west.

Australia's largest infrastructure project marked a milestone on Wednesday when a road header bored through rock to connect two sections of tunnels which, by 2019, will have thousands of vehicles passing through it every day.

About 70 per cent of the excavation for the 5.5-kilometre motorway tunnels has now been completed.

Premier Gladys Berejiklian said the tunnel breakthrough was a "significant milestone" and demonstrated how much progress had been made in the motorway project.

"It is a wonderful reminder to all of us to what we have to look forward to when this part of the project is completed in 2019. It means less travel time for people in western Sydney," she said.

"It means we are able to take traffic from above ground and move it underground."

The three-lane tunnels in each direction from Haberfield to Homebush will connect to a widened 7.5-kilometre section of the M4, which was opened to traffic early this month.

Once a month-long "toll holiday" ends on August 15, motorists will pay distance-based tolls of between $1.77 and $4.26 for cars on the widened M4.

The tunnels under construction between Homebush and Haberfield will allow motorists to bypass a major traffic bottleneck at Concord where the M4 ends at Parramatta Road.

The Minister for WestConnex, Stuart Ayres, said more than 200,000 vehicles had used the first section of the New M4 since it opened two weeks ago.

"We are not funnelling people onto the M4. They get a choice of a free road. But our offer to them is faster travel times and every person on the M4 can see that today," he said.

An environmental impact statement for the M4-M5 link – the final stage of WestConnex costing $$7.2 billion – is due to be released next month.

A major interchange at Rozelle in the inner west and tunnels linking the M4 and M5 will be carried out as separate packages.

Mr Ayres said work was still under way on the final designs for the interchange at Rozelle, declaring it too early to say what impact construction would have on thoroughfares such as Victoria Road.