In the wake of the decision, the Greens labelled the planning process for Australia's largest motorway project a sham and called for civil disobedience to stop it.

"The time for civil disobedience to stop WestConnex is now," the Greens' member for Newtown, Jenny Leong, said.

"It's now clear that the planning process has been a sham and that's why we are encouraging the community to step it up."

The approval for the so-called New M5 came as about a dozen residents occupied NSW Planning's main office in Sydney's CBD on Thursday for about two hours to protest against the controversial motorway project. They were eventually ejected from the building without any arrests.

Several months after public submissions closed, Planning Minister Rob Stokes has given the green light for construction of twin nine-kilometre tunnels and a motorway interchange at St Peters, in the inner west.

The second stage of the tolled motorway project is scheduled to open to motorists in 2019. As many as 143 properties are targeted for partial or full acquisition to allow for the New M5, according to planning documents. The full 33-kilometre WestConnex is to open to traffic in 2023.

As part of the approval, Mr Stokes has set a number of conditions on the New M5 in response to residents' concerns. The conditions include construction of a new "open space" south of Sydney Park in St Peters. A land bridge is to connect the parks. However, the government has yet to finalise details on the new park, such as its exact size.

Residents opposed to the project also described the planning process and conditions placed on the New M5 as a farce.

"We have seen this before. As far as we are concerned. they have proved that months of consultation are virtually meaningless," WestCONnex Action Group spokeswoman Pauline Locke said.

"They got a near record number of submissions. It is astounding that the Department of Planning is saying that process is perfectly OK."

The government also promised new and upgraded cycle and pedestrian paths connecting Sydney Park, Simpson Park, Camdenville Park and the interchange at St Peters.

It said "strict air quality limits" for the New M5 tunnels and ventilation shafts would be imposed, and a committee established to help decide locations of monitoring stations.

Roads Minister Duncan Gay said initial construction of the second stage could begin within weeks, pending approval from the federal government.

"One hundred thousand motorists use the current M5 East every day and face a constant battle to get through what has become a frustrating car park. By doubling its capacity with new nine-kilometre twin tunnels, they'll have a much faster and less stressful journey," he said.

"Sydney motorists have been crying out for this project ever since the opening of the original M5 East – we are cracking on with delivering it."

Almost 12,900 submissions from residents, councils and community groups were lodged in response to the environmental impact statement for the M5 East tunnels between St Peters and Beverly Hills, in the city's south-west. Almost all were opposed.

The high number of submissions was surpassed only by those lodged in the 1990s over construction of the M2 motorway in Sydney's north-west, which attracted about 14,000.

Sydney lord mayor Clover Moore said it was "disingenuous in the extreme" for the government to tout extra open space at St Peters when it would be dangerous for children and elderly people to be exposed to the air around it.

"Air quality, even at existing levels, is close to the allowable limits, and it's only likely to get worse," she said.

Cr Moore said the Planning Minister had not responded to the City of Sydney's concerns about the tens of thousands of extra cars that would be funnelled onto streets in the central city each day.

The WestConnex project comprises three sections: the new M5 East; a widened M4 motorway and new M4 East tunnels; and a new M4-M5 tunnel linking the two.

In February, Mr Stokes approved the M4 East section between Homebush and Haberfield. In approving the project, he said at least one lane each way on Parramatta Road needed to be reserved for public transport. Giant boring machines are due to begin work on the 30-metre wide tunnels for the M4 East by the middle of this year.

The WestConnex motorway project relies on funding from the state and federal governments, as well as on tolls from motorists when it eventually opens to traffic.