Tolls for Sydney's F6 extension: NSW govt

Motorists travelling to and from Sydney's south are being promised a 15-minute shorter ride via a new underground tunnel - but they'll have to pay for it.

The NSW government on Tuesday released plans for the four-kilometre F6 motorway extension, running from the M5 at Arncliffe to President Avenue in Kogarah.

The project is set to cost up to $2.6 billion and will be tolled.

Drivers will pay an estimated $1.77 each way for cars to use the new section, but that rises to around $4.53 when they continue along Westconnex.

"We want to be up-front with the community now and say this is one of those roads which will be tolled, but as always there will be a free alternative," NSW Roads Minister Melinda Pavey said.

She denied it was simply a "funnel" into Westconnex, describing it as an important "add on" to Sydney's road network.

"A network that we're creating, that we're delivering with the support of tolls so we can also spend money on public transport," she told reporters.

The government says the tunnel will let drivers bypass 23 sets of traffic lights and take more than 10,000 vehicles off the often-clogged General Holmes Drive.

But the NSW Greens say it's a "huge burden" on people and the environment.

"This government is locking Sydney into an ever tightening noose of motorways, where people will have no escape from paying thousands of dollars in tolls," the party's roads and transport spokeswoman Mehreen Faruqi said in a statement.