Raised Warragamba Dam wall risks ‘irreversible harm’ to Aboriginal cultural heritage, NSW report says

The $1.6bn plan to raise the Warragamba Dam wall would cause "irreversible harm" to Aboriginal cultural heritage, a New South Wales government-commissioned report has found.

Suggesting it is open to Unesco to "provide for modifications to the boundaries of World Heritage properties", the report said the controversial project could also lead to hundreds of sites of Aboriginal cultural heritage suffering increased flooding, including the "total loss" of some sites.

Prepared on behalf of the project's proponent, WaterNSW, the report was produced by consultancy firm Niche after an initial assessment was savaged by Indigenous groups and government agencies.

The updated report, obtained by the Guardian, concedes that if the New South Wales government goes ahead with its controversial plan to raise the dam wall there would be "unavoidable" and "irreversible" harm to sites of Aboriginal heritage.

The project, it found, would have a "deleterious effect" on cultural heritage values and cause "additional injury to the wounds of previous dispossession and loss".

"The project will result in cumulative harm to the intangible values of the cultural landscape through extension of previously unmitigated impact on cultural values from the construction of the Warragamba dam and flooding of the Burragorang valley and its tributary valleys," the report stated.

"The further flooding of the Burragorang valley will contribute to irreversible harm to the cultural and spiritual connection that Aboriginal people hold to this part of the country, their heritage and the cultural landscape and will obscure the tangible aspects of the creation stories associated with the Burragorang such as the Gurrangatch and Mirrigan story."

Those harms, it found, could not be directly mitigated. Instead, it recommended "increasing the broader community's knowledge of Aboriginal history in the Warragamba area" as a way of increasing "intergenerational equity".

While the report found that more than 250 sites of cultural heritage would be affected by the project - including dozens of new sites including forms of rock art "that are not commonly represented regionally" - it also conceded there was likely to be far more culturally significant artefacts outside its survey area.

The damning report comes amid increasing questions about the controversial project.

The premier, Dominic Perrottet, earlier this month moved to stop the plan being challenged in the courts by designating it as a critical state significant infrastructure project, but it has been the subject of fierce criticism from environment and Aboriginal stakeholders.

On Friday a document obtained by the Wilderness Society via freedom of information laws also raised concerns about the wall raising's impact on water quality.

The internal Sydney Water document stated that "poorer quality water" stored in the dam "for extended periods" would risk the agency's water filtration plant's ability to "operate at capacity and increasing chances of failure to supply water and the need to boil water".

The Coalition in NSW has sought to make the dam wall an election issue with voters in western Sydney after a spate of flood events in the Hawkesbury-Nepean area.

While expert reports have found it would mitigate flood risk, many experts believe it would be of limited value with potentially disastrous environmental effects.

The Guardian understands WaterNSW officials believe it is unlikely to be ready for approval until after the March election.

Unesco previously raised concerns about the projects impact on the world heritage-listed Blue Mountains area. While the draft report obtained by the Guardian insists increased inundation as a result of the raised wall would not have a "material effect" on the area's value, it concedes it had "the potential to cause permanent harm through physical impacts to the sites and potential alterations to the waterways and ecology of the project area".

The NSW government is also banking on a 50-50 funding split on the project with the commonwealth, which has yet to indicate support for the project.

In a statement, a spokesperson for Water NSW said it would not comment on the details of the report "on recognition of important cultural sensitivities associated with the document".