A draft environmental impact statement confirms the NSW government is planning to raise parts of the Warragamba Dam wall higher than was admitted publicly, meaning a much larger area of the Blue Mountains could eventually be inundated.
Critics say the plan puts at risk a "secret biodiversity wonderland" that some experts have labelled Sydney's Jurassic Park.
The coalition government has repeatedly said the dam wall needs to be raised just 14 metres to help prevent flooding of the Hawkesbury-Nepean Valley below the Blue Mountains.
But AAP revealed in March the Berejiklian government was actually going to raise each end of the wall - the dam abutments - by 17m so it could easily be modified in the future to hold back even more water.
The secret documents revealed the wall would be "structurally" raised 17m but "operational" at 14m.
Now the draft EIS, seen by AAP, confirms the plan involves "raising the wall by 14m with abutments for 17m".
The statement - which is due to be made public later this year - isn't assessing what would happen upstream if the wall was raised 17m.
That additional three metres could cause significantly more damage to threatened flora and fauna in the heritage-listed Blue Mountains, Australian National University water expert Jamie Pittock has told AAP.
"It would mean tens of kilometres of more inundation," the professor said in March.
Premier Gladys Berejiklian was quizzed about the proposed height of the dam wall at that time.
"I ask you to rely on what's in the public domain - that's what we've announced (and) that's what's happening," she told reporters.
The federal government in September 2017 told the UNESCO World Heritage Committee the wall would be raised 14m.
Members of the Give a Dam community group are this week travelling to UNESCO headquarters in Paris to brief the committee of the project's potential negative impacts.
"If the Australian government allows the NSW government to continue down their destructive path of raising the dam wall, UNESCO will rightfully come down on Australia like a tonne of bricks," campaign director Harry Burkitt told AAP in a statement.
"It's now prime time for federal politicians to listen to the thousands of Australians opposing the project and rule out supporting the dam themselves."
Infrastructure NSW on Tuesday was asked if the government was planning to raise the wall's operational height to 17m in the future.
The agency replied that "the NSW secretary of the Department of Planning and Environment has required the project to be designed, constructed and operated to be resilient to the future impacts of climate change".