$18b Sydney F6 extension price 'rubbish'

The $18 billion price tag on the proposed extension of the F6 motorway in southern Sydney is "rubbish" but the road link is still preferred over the cheaper, alternate rail upgrade, the NSW government says.

According to confidential cabinet documents released to the media on Wednesday, the construction cost of the 32km road has been estimated at $14.5 billion, with operation and maintenance costs estimated at $3.5 billion.

The hefty $18 billion total cost is about $1 billion more than was set aside to build one of Australia's biggest road projects, WestConnex.

However, Transport Minister Andrew Constance denies the Sydney to Wollongong link will cost that much and insists no price has been confirmed because the project is still in its early stages.

"The $18 billion figure is rubbish," Mr Constance told reporters in Sydney.

"You don't know the cost of a project until you finalise the alignment."

Another report leaked last week showed the cost to upgrade the South Coast rail line - which also connects the city to the Illawarra region - at $3.6 billion.

The estimate, which is substantially less than the proposed F6 extension, would cut the Sydney to Wollongong journey to about one hour, the report said.

Mr Constance insists the F6 motorway extension is the better option.

"There are 37,000 motor vehicle movements to the Illawarra into town every day versus about 2500 rail passengers," he said.

"People want to work and live locally in the Illawarra and if we can reduce people movement to the city, that's the preference."

The leaked document also makes mention of a controversial proposal to build part of the extension through Sydney's heritage-listed Royal National Park.

NSW Greens MP Mehreen Faruqi has criticised the "obscene" project, saying public transport upgrades would slash the time people spend commuting to the city from the South Coast at a smaller price.

"I think we have reached peak toll road madness with this F6 proposal," she said in a statement.

"Eighteen billion dollars of taxpayers' money for yet another toll road, potentially through a National Park, is just obscene."