A new $5 billion tunnel to be built as part of the WestConnex​ motorway project will carry fewer vehicles when it opens than the long-troubled Cross City Tunnel, government figures show.

And in extraordinary details released on Friday, official predictions show that when a second M5 East tunnel is completed in four years, fewer cars will be using the two M5 East tunnels than currently use the one.

That is because tolls of about $6 a trip will be charged for the first time on the existing M5 East as well as the new tunnel that runs alongside it from Beverly Hills to St Peters.

In response, motorists are expected to pour onto alternative free routes such as Canterbury, Stoney Creek and Forest roads.

But the government insists the new motorway is needed to expand the city's road capacity to cope with a growing population.

An environmental impact statement released on Friday into the M5 East component of WestConnex predicts average weekday traffic on the new nine-kilometre tunnel will total about 29,500 vehicles in 2021, before rising to 34,500 a decade later.

In comparison, about 37,000 vehicles a day pass through the Cross City Tunnel, which connects Sydney's eastern suburbs with Darling Harbour and the inner west. That 2.1km tunnel was twice placed in the hands of receivers before Australia's tollroad king, Transurban, bought it last year for the fire-sale price of $475 million.

While the new tunnel will ease congestion significantly on the existing M5 motorway, the environmental impact statement shows traffic on Stoney Creek Road is predicted to surge by 43 per cent by 2031.

"Traffic is forecast to divert off the motorway network to use other routes ... outside the peak traffic periods to avoid the proposed toll," it said.

The funnelling of vehicles onto Stoney Creek Road is expected to happen in off-peak periods because capacity on that road is limited during rush hour.

Labor MP for Heffron Ron Hoenig​ described the $16.8 billion WestConnex project as the most "expensive traffic jam in the country's history", accusing the government of "turning our local roads into glorified parking lots".

While reducing the likelihood of accidents elsewhere, the latest report noted that a forecast increase in traffic on Euston Road near Sydney Park in the inner west "is likely to cause an increase in the total number and cost of crashes" on that stretch of road.

At St Peters, the modelling says there will be reduced congestion and delays. Though whether Newtown will become the "Nirvana" predicted by Roads Minister Duncan Gay is debatable.

Travel times are predicted to drop from about nine minutes to eight minutes during the morning rush hour through St Peters.

The detailed forecasts show motorists who use the new M5 East tunnels between St Peters and King Georges Road will have an average travel time of nine to 10 minutes during morning and afternoon rush hours by 2031.

It also forecast a significant fall in traffic on the existing M5 East from about 116,000 to about 69,000 vehicles a day at its western end by 2021. It means average peak hour travel times on the existing M5 East are forecast to drop from about 20 minutes to 10 minutes.

Greens MP for Newtown Jenny Leong said the "spaghetti junction" created at St Peters would turn the suburb into a concrete jungle and force authorities to create clearways along King Street in Newtown which will threaten the viability of local businesses.

Ms Leong also criticised the government for releasing the environmental impact statement just as councils were to break for Christmas, reducing the chance of a thorough review of hundreds of pages of documents. "It's completely outrageous," she said.

However, Mr Gay said the new M5 motorway would "deliver a host of benefits" including cutting an eastbound morning peak trip down from 20 minutes to just 11 minutes.

He said WestConnex needed to be "considered in its entirety and not as a piecemeal approach" and that without additional motorway capacity travel times would continue to worsen.

"Comparing an individual stage of what is a massive world-class motorway spanning 33 kilometres to the Cross City Tunnel is like comparing apples with oranges," Mr Gay said.

Without WestConnex, the government said forecast demand on local roads at St Peters would be up to 30 per cent higher by 2031.

But Labor's roads spokeswoman and member for Strathfield Jodi McKay said the government might be obliged to rethink its tolling strategy given the traffic forecast for the new M5 East link was less than the Cross City Tunnel.

The release of the lengthy report comes a week after the cost of WestConnex ballooned by $1.4 billion to $16.8 billion. Its cost has climbed steadily since a $10 billion project was proposed by Infrastructure NSW in 2012 due to significant additions.

The federal and state governments have committed $4.3 billion to WestConnex. Sydney Motorway Corporation, the organisation delivering the project, plans to raise billions of dollars by selling the rights to toll early sections of the motorway.

In what it described as an "acid test" for interest in the project from the private sector, WestConnex raised $1.5 billion of debt last week from Commonwealth Bank, Westpac, National Australia Bank and France's Credit Agricole.