The NSW government's metro rail project under Sydney Harbour is reportedly more than $4 billion over budget, prompting Labor to question whether the coalition knew of the blowout before the 2019 state election.
The rail line, which will link Chatswood to the CBD and extend to Sydenham and Bankstown, was budgeted to cost up to $12.5 billion.
However a confidential internal review completed about 18 months ago by Sydney Metro, which is building the railway, suggests there's been a $4.3 billion blowout, according to Nine Newspapers.
That could push the bill as high as $16.8 billion.
Issues include cost overruns for new trains and systems, excavation of tunnels and underground stations and construction at Sydenham and Central stations.
Sydney Metro, in a statement provided through the NSW transport department, said the project was on schedule to open in 2024 and its final cost wouldn't be known until the commencement of services.
"This is the largest public transport infrastructure program in the nation's history," a Sydney Metro spokeswoman said on Tuesday.
NSW Opposition Leader Jodi McKay suggested the government had hidden its knowledge of the increased costs during the March 2019 state election.
"The government obviously knew about this and didn't tell us ... this document dates back 18 months so they knew this project had blown out when they went to the election," Ms McKay told reporters on Tuesday.
"The last budget did not reflect a blowout in the cost of this project."
Ms McKay warned the government may resort to privatisation of assets and services if they could not obtain the extra $4.3 billion from the NSW Treasury.
Sydney's first driverless northwest metro line, between Rouse Hill and Chatswood, opened in May 2019. Plans are in place to extend the network to Parramatta, Westmead and the planned western Sydney airport.