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.
The business case for WestConnex, released in November 2015, also states that that the gateway "falls as part of the stage two project" and will open "at the latest" by 2023.
However, WestConnex Minister Stuart Ayres told Parliament on Wednesday that the gateway was not part of WestConnex, and that the $16.8 billion for the toll road included an "allocation" of $800 million for the connection to the airport.
Quizzed again in Parliament on Thursday, Mr Ayres clarified that "one thing I did not do yesterday was say that the Sydney gateway project would cost $800 million".
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.
But, earlier on Thursday, Premier Gladys Berejiklian told reporters that the gateway to the airport "feeds into the [WestConnex] project but it's not part of the project – it never was".
And she disputed the fact that the estimated cost of the gateway had increased "when the final business case hasn't happened yet".
Under the most recent plans for the gateway, a motorway of two lanes each way would split much closer to St Peters in Sydney's inner west than had been originally envisaged. One arm of the new roadway would connect to the international terminal and the other to the domestic terminals known as T2 and T3.
Those plans would also require moving a rail freight line near the northern boundary of the airport about 500 metres north. The single rail line connects Port Botany to Sydney's west.
Mr Ayres said the gateway would not have the same distance-based tolling regime as WestConnex and was "not being delivered by the Sydney Motorway Corporation", a body set up by the government to oversee Australia's largest toll road project.
The latest cost estimate highlights how WestConnex has morphed from a $10 billion roadway, when proposed by Infrastructure NSW in 2012, into a project now at risk of blowing out to as much as $17.8 billion.
That cost does not include some major roads planned by the government to connect to the project such as an upgraded road corridor from Alexandria to Moore Park.
The story $1 billion cost blowout in Westconnex gateway project to Sydney Airport appeared initially in The Sydney Morning Herald.