The Blue Mountains' world heritage listing could be threatened by a NSW government plan to raise Warragamba Dam wall.
The International Council on Monuments and Sites, which advises the UNESCO World Heritage Committee, has cautioned both the federal and NSW governments about the plan to raise the wall by about 14 metres.
The council claims the proposal could flood up to 1000 hectares of the Greater Blue Mountains world heritage area and 3700 hectares of surrounding national park.
ICOMOS Australia president Ian Travers says if it goes ahead the World Heritage Committee could place the area on the List of World Heritage in Danger.
"It's worth recognising how much effort the commonwealth government went to with the Great Barrier Reef to prevent it from going on the danger list," Mr Travers told AAP on Wednesday.
"It's a very bad look."
ICOMOS Australia, in a letter sent to NSW Environment Minister Gabrielle Upton and federal Environment Minister Melissa Price in October, says the Warragamba proposal does not comply with Australia's obligation to protect and conserve the heritage of the Greater Blue Mountains.
The proposal has the potential to affect the integrity of the area and its value as a world heritage property, the letter says.
It calls upon Ms Upton and Ms Price to urgently intervene and consider alternatives.
"If the government is prepared to sign legislation that is directly focused on damaging a world heritage area, that is going to be perceived very badly by the World Heritage Committee," Mr Travers said.
He also called on the removal of NSW legislation permitting flooding of the national park through a "controlled release" which then allows the dam wall to be raised.
Community group Give A Dam spokesman Harry Burkitt said the NSW government was seeking to destroy one of the most protected areas in Australia.
"It has become clear the NSW government has no regard for protecting the state's natural assets," Mr Burkitt said in a statement.
The NSW government believes the Warragamba proposal will reduce and manage flood risk in the Hawkesbury-Nepean Valley.
An environmental impact statement for the project is yet to be released which will then be subject to state government approval before it's given the green light.
The NSW government has been contacted for comment.
Australian Associated Press