New confidential estimates on the F6 freeway extension through southern Sydney price the road at around $18 billion, about $1 billion more than WestConnex.

Key points: Extending the F6 through the Royal National Park is listed as a preferred option in the documents

Extending the F6 through the Royal National Park is listed as a preferred option in the documents The Greens say SouthLink's $18 billion price tag is "obscene"

The Greens say SouthLink's $18 billion price tag is "obscene" Last week, the ABC revealed a proposed rail tunnel from Thirroul to Waterfall would cost $3.6 billion

A Reference Design Estimate Report marked "Cabinet in Confidence" and viewed by ABC News reveals the detailed costings.

The Final Business Case (FBC) is the last step in the appraisal of the F6, also known as SouthLink.

It outlines four stages of the 32km extension, starting with Stage A in St Peters through to Stage D, which ends at Waterfall.

The report lists as the preferred option two separate tunnels between Heathcote and Waterfall, combined with a road through the Royal National Park.

"A separate motorway will extend through the Royal National Park from south of Farnell Avenue to south of Waterfall," the report reads.

The SouthLink toll road could be built through the Royal National Park. ( ABC News: Justin Huntsdale )

The construction costs are estimated to be $14.5 billion, but when ongoing operation and maintenance costs for 35 years totalling $3.5 billion are included, the total cost increases to $18 billion.

Roads Minister Melinda Pavey would not acknowledge the detailed costings.

"We're still in the very early stages, so no decision has been made regarding the final alignment," she said.

"There is no decision on final cost."

Greens slam blueprint as 'obscene'

The NSW Greens transport spokeswoman, Dr Mehreen Faruqi, called the F6's price tag "madness".

"$18 billion of taxpayer's money for yet another toll road, potentially through a National Park, is just obscene," she said.

The F6 leak comes after last week's revelations about detailed costings for an upgraded rail link between Sydney and Wollongong.

Transport bureaucrats costed the South Coast rail upgrade, including a tunnel from Thirroul to Waterfall, at $3.6 billion.

It would cut the trip to Sydney by 24 minutes for one fifth of the cost of the F6 upgrade.

Andrew Constance said money was being spent on roads and rail in the Illawarra. ( AAP: Dan Himbrechts )

But NSW Transport Minister Andrew Constance rejected suggestions the region was missing out on its fair share of infrastructure funding.

"Brand new trains for the people of the Illawarra, massive improvements to the rail network in Sydney, I mean $1.5 billion invested in trying to improve the speed of trains from the Central Coast, Illawarra, the Blue Mountains to try to speed up the travel times to Central," he said.

Mr Constance said a Thirroul-to-Waterfall railway tunnel was not feasible, but Wollongong Labor MP Ryan Park urged the Government to nominate the line as a priority project so it could receive federal funding.

"The way this can be done is by nominating the South Coast line as a priority project for a share of business case funding under the Federal Government's National Rail Program," he said.