But opponents of the highly contentious 33-kilometre motorway project have criticised plans for green space beneath the interchange as creating "the world's worst park".

Ms Berejiklian and WestConnex Minister Stuart Ayres continued to spruik the benefits of the $16.8 billion mega toll road as they offered a glimpse of above-ground construction at the interchange.

The road and tunnel network for WestConnex, described as the nation's largest infrastructure project, is aimed at linking Sydney's existing motorway network and easing congestion.

Ms Berejiklian said excavation of two nine-kilometre tunnels that will run from Kingsgrove to St Peters were 75 per cent complete and would help double the capacity of the clogged M5 corridor.

The interchange is set to open in early 2020.

"This project's running on time, on budget," Ms Berejiklian said, as cranes swung girders for bridges at the site into place on Tuesday.

"This is a significant milestone. It's really a massive project with great, great progress being made."

Ms Berejiklian said the motorway would slash travel times from the city's south-west to the CBD.

"If you're coming from Liverpool you'll save about half an hour, if you're coming from [Oatley area] you'll save about 15 minutes."

She said eight or nine bridges set to be constructed at the junction would "really give life to this part of the inner west" and emphasised some of the former landfill site would be green space.

"We are appreciating what quality of life means for people in this area. It means less travel time, less traffic, it also means more open space."

The size of the interchange project has faced strident opposition from the inner west community, who have also voiced concerns over construction noise, increased traffic and acquisition of homes.

"I tend to refer to that as the world's worst park," Inner West councillor Pauline Lockie, a vocal WestConnex opponent, said of the promised green space.

"We have serious doubts about whether this green space will actually be delivered.

"A lot of that space is going to be dead space under these huge flyovers. Nobody's going to want to throw a picnic rug under those things."

Ms Lockie also said the expected completion date for the interchange had previously blown out, with promises it would be completed next year.

Ms Berejiklian said she acknowledged the ongoing disruption and disturbance caused by the project, but said overall WestConnex would be "good news" for the local community.

"When this project has finished, not only will you have less traffic on local roads, because traffic will be underground, but you're also going to be given open space you didn't have before.

"Former industrial sites, former landfill sites are going to be converted into beautiful green parks for the community to use."

Ms Lockie said residents were also concerned their streets would become congested from induced demand for the motorway or "rat-runs" for drivers who wanted to avoid tolls.

Mr Ayres said he was "working with RMS around the road integration right across the WestConnex project" in an effort to minimise congestion on roads surrounding the interchange.

"This is the road project that everyone in Sydney knows we need."

He said work would soon start to surface and fit out the New M5 tunnels. About 2000 workers are on-site.

Meanwhile, the competition watchdog is deciding whether to block private toll road operator Transurban from buying a controlling stake in the motorway.

The Australian Competition and Consumer Commission last month delayed a review decision until September 6.