The estimated cost of the Sydney gateway project connecting the WestConnex tollroad to the city's airport and port has more than doubled to as much as $1.8 billion, a leaked document shows.
The blowout of up to $1 billion in the expected cost of the motorway link to the airport, from $800 million, threatens to push the total bill for WestConnex to $17.8 billion.
Details of the escalation are contained in a Transport for NSW document obtained by Labor. It estimates the cost of the gateway at between $1 billion and $1.8 billion, and shows that the project has received a "health check" from Infrastructure NSW.
The gateway was one of the original justifications for WestConnex but there remain few substantial details on its likely final shape years after the government committed to the controversial motorway.
Labor accused the government of a "shameful cover up of a cost blowout", pointing out that the gateway project was mentioned 45 times in the business case for WestConnex.
"They are doing everything they can to keep this project at $16.8 billion when it has in fact gone beyond that," Labor's transport spokeswoman, Jodi McKay, said. "This is part of WestConnex. The entire premise for WestConnex was to get to the airport and port."
Ms McKay said the state's senior transport bureaucrats had also confirmed it was part of WestConnex during budget estimates hearings in August.