Transport Minister Jo Haylen is blaming Mark Speakman for the 333 per cent jump in the cost of building new ferry wharves at Kurnell and La Perouse despite him saying the process was at "arms length from government ministers".
Ms Haylen told State Parliament it was the Opposition Leader's "pet project" and "his fingerprints are all over [it]".
Work is due to start on construction within the next few weeks, with completion expected in late 2024.
The new wharves will allow the restoration of a ferry service after 50 years.
Ms Haylen revealed in answer to a question on notice from Mr Speakman that, when the project was conceived in 2018, the estimated cost was $18 million.
However, Transport for NSW informed the Minns government in its first weeks in office the latest estimated cost of the project was $78 million, up from $65 million in June last year.
"Cancelling the project is estimated to cost at least $46 million and, with 98 per cent of the project's value locked into contracts, the government is left with no option but to continue with this project," she said.
Ms Haylen was later speaking during Question Time about the Minns government bringing back the Freshwater class ferries to Sydney Harbour when she changed tack.
"We should turn our attention to a very different ferry project, the Kamay ferry wharves," she said.
"That pet project was the brainchild of the member for Cronulla. Back in 2019 it was costed at $18 million, two years later the cost jumped to $34 million, six months later it jumped again to $56 million and in June last year it jumped to $65 million.
"But that is not all. The total cost of the pet project of the member for Cronulla is now a whopping $78 million. That is a 333 per cent increase on what members opposite said it would cost. But it gets worse.
"Building nothing at all, to stop it completely, would cost taxpayers $46 million.
"But what does the member for Cronulla have to say about this? 'It has nothing to do with me. That was all done at arm's length from government.'
"The truth is that the Leader of the Opposition's fingerprints are all over this project."
Mr Speakman attributed the cost blowout to "extraordinary" pandemic-related increases in construction costs and the time taken for approval by the federal government.
"The cost of about $65 million has been on the public record, and been budgeted for, since the 2022-23 NSW budget last June (noting also that there was a federal contribution under the joint funding of Kamay projects announced in 2018)," he told the Leader last week.
"I don't recall Labor ever complaining about that before now.
"The latest figure of $78 million was apparently calculated during the election caretaker period, so I don't have access to the details of that."
Mr Speakman said "the procurement contract was the result of going to market under a tender process and was handled at arms-length from government ministers, as major NSW Government procurement contracts generally are".
"The environmental approval by the federal government to deliver the project was only granted in March 2023 during the caretaker period.
"I understand that extraordinary COVID-related increases in construction costs, and therefore tender prices, are the reason for overall cost increases."