The final piece of Sydney's WestConnex puzzle has been placed, after the New South Wales Government today announced stage three of the controversial project had been approved.

Key points: The $7 billion link runs underground for about 8 kilometres

The $7 billion link runs underground for about 8 kilometres It will connect the M4 at Haberfield to the M5 at St Peters, via another interchange at Rozelle

It will connect the M4 at Haberfield to the M5 at St Peters, via another interchange at Rozelle The NSW Government claims it will save "hours of travel time" each week

It means a tunnel will be built connecting the M4 at Haberfield to the M5 at St Peters — a development WestConnex Minister Stuart Ayres described as being "like the Sydney Harbour Bridge".

It will create a non-stop bypass of Sydney's CBD and inner-west, slashing travel times.

However, the proposal has also attracted significant opposition.

This time last year, Sydney Lord Mayor Clover Moore briefed Premier Gladys Berejiklian on a set of alternatives at a meeting.

The $17 billion WestConnex development has been previously described by the NSW Government as the world's biggest road project.

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

Mr Ayres said the tunnel was crucial to the city's transport future.

"Like the Sydney Harbour Bridge did for the North Shore, the M4-M5 Link will bridge a major gap in the road network, creating a non-stop underground western bypass of Sydney's CBD, slashing travel times and delivering over 18 hectares of open space for local communities," he said.

In August 2017, the Environmental Impact Statement (EIS) for WestConnex stage three said the project would slash travel times from Sydney's western suburbs to the CBD.

It argued the 55-kilometre trip from Penrith to the CBD could cost drivers $22 today, but when WestConnex was finished, the tolls would be capped at $8.60 for the same journey.

A map showing all three stages of the WestConnex project. ( Supplied: NSW Government )

Final stage 'hasn't even been designed', Labor says

Labor's roads spokeswoman Jodi McKay accused the Government of trying to avoid public scrutiny by announcing the approval on a Friday afternoon.

"This is the largest infrastructure project in the states history, the final stage of it, and no one is around to actually enlighten people about this project," she said.

Ms McKay said the Government rushed the approval process for the Rozelle interchange and ignored community concerns because it is seeking to sell the Sydney Motorway Corporation.

"This is an extraordinary situation given the Government has approved something that hasn't even been designed," she said.

"There has been no community consolation, there has been no transparency yet suddenly today we find out it's been approved."

Project settled behind closed doors: Greens

Reacting to the approval of West Connex's final stage, Inner West Mayor Darcy Byrne said: "What we've just discovered is that West Connex stage three was secretly approved 10 days ago behind closed doors".

"No wonder the Government is ashamed of this wasteful project," Mr Byrne said.

"Throughout the Inner West we're now going to see more smoke stacks and the mother of all rat runs.

"With the demolition of homes, the secrecy behind the West Connex project, people know this Government has it in for us and...[stage three] will be worse than anything we've seen yet."

Mr Byrne said he wanted "to see the Government invest in a proper solution to modern congestion which is of course...public transport."

'We need to see transparency'

State Greens MP and member for Newtown, Jenny Leong, said "the arrogance of the Berejiklian Government knows no bounds".

"We have seen tens of thousands of submissions from the community and experts opposing this (project).

"The public appetite is there... to open up the books.

"The community have put their concerns front and centre in this planning process."

Ms Leong said the NSW Greens were pushing for more transparency.

"We are urging the NSW Labor Opposition and the crossbenchers of the upper house to support the Greens' call for the exposure of the papers. We need to see transparency and accountability."