Docs reveal another WestConnex blowout

The cost of the Sydney Gateway extension of the major WestConnex road project could blow out by up to $1 billion, leaked documents suggest.

Sydney's WestConnex road project could blow out by up to $1 billion, leaked documents suggest. (AAP)

Leaked NSW government documents suggest the cost of a major WestConnex extension could blow out by as much as $1 billion.

The documents, from April, list the estimated total cost of the "WestConnex Sydney Gateway" as being between $1 billion and $1.8 billion, despite an initial estimate of just $800 million.

The cost of the entire WestConnex project was revised to $16.8 billion in late 2015 - $6.8 billion more than first estimated.

The government argues the Sydney Gateway is not part of the WestConnex project despite information of the extension featuring prominently on the WestConnex website.

NSW Premier Gladys Berejiklian on Thursday said the Gateway project, which will extend the road to the airport and Port Botany, was never part of WestConnex.

"Obviously it feeds into the project, but it's not part of the project, it never was and we've been very clear about that," she told reporters.

But information on the WestConnex website says the project will "provide a connection to Sydney Airport and Port Botany via Sydney Gateway".