A spaghetti junction of motorway ramps will be built just south of Sydney Park in inner Sydney, but part of the area will be returned to the public as parkland.

Roads Minister Duncan Gay announced on Wednesday a consortium of Leighton, Dragados and Samsung was the preferred tenderer for the $5 billion second stage of the WestConnex motorway, a new M5 tunnel from Beverly Hills to St Peters to open in 2019.

Preliminary details of the motorway interchange at St Peters, also released on Wednesday, show a criss-crossing network of above-ground ramps at a site previously used as a dump.

But when the entire WestConnex motorway is complete, some time in 2024, the idea is to turn segments of the site into a park, connecting to the existing Sydney Park by new walkways and cycleways.

"I am as proud as hell of this project," Mr Gay said. "It is going to make a hell of a difference to this city," he said.

Mr Gay said the project was called the M5 tunnel, to signal its improvement over the old M5 tunnel.

"The old one was crappy and redundant from the day it was opened," he said. "This one will open with three lanes built, marked for two lanes, from day one."

The project, however, has been heavily criticised by local residents and politicians, including the Lord Mayor, Clover Moore. They fear it will increase traffic on inner city streets and encourage more car use in Sydney.

"Even I was staggered by how big that is," Pauline Lockie, a St Peters resident and campaigner against WestConnex, said of the images of the interchange.

"All this for a toll-road where they haven't been able to prove the benefits they keep going on about will be there," she said.

The $15.4 billion WestConnex project consists of three stages: a widened M4 motorway and M4 East tunnel from Homebush to Haberfield, the new M5 tunnel from Beverly Hills to St Peters, and a third stage connecting Haberfield to St Peters via Rozelle.

Mr Gay reiterated his comment that Newtown, just north of the new motorway interchange, would become a nirvana once the third WestConnex stage was built.

"Those people that put effigies of me in Newtown should be treating me as a deity," he said.

Chris Swann, the project director for the new M5 tunnel said about eight hectares of parkland would be added to the existing 40-odd hectares of parkland at Sydney Park.

Mr Swann said there would be "up to seven kilometres of new cycleways and pedestrian walkways" that would provide a direct link from Mascot train station to Sydney Park.

The project will, however, slice a 12 metre strip from Sydney Park, already owned by Roads and Maritime Services, to widen nearby Euston Road.

Ventilation stacks, about 35 metres high, will be built at Beverly Hills, Arncliffe and St Peters.

The contract with Leighton, Dragados and Samsung has not yet been signed, but the government intends to sign it before an Environmental Impact Statement is approved. That statement will be released later this year, as will a separate EIS for the M4 East tunnel component near Parramatta Road.

When the project is open in 2019, motorists will have to pay a toll of about $6 to use the road, and will also have to pay that toll to use the existing and free M5 East.