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

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.

Operation and maintenance costs of the freeway, which will connect Sydney to Wollongong, has been estimated at $3.5 billion, the document says.

The revelations come after another report leaked last week showed the cost to upgrade the South Coast rail line - which also links 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.

A spokesman for Roads Minister Melinda Pavey says the motorway extension is still in an early development stage.

"No decision has yet been made regarding the final alignment or form of the proposed F6 Extension, therefore it is too early to speculate on the final cost," he told AAP in a statement on Wednesday.

"The community will continue to be kept informed at key stages of the project."

The Berejiklian government allocated $15 million in June's budget for planning work on the F6.

A further $20 million has already been provided to carry out geotechnical testing and other development work.