The August 2020 briefing note identified that the potentially billion-dollar plan to lift the wall 14-plus metres had met “clear opposition from the Aboriginal community”.

The latest leak adds to a growing list of state and federal agencies to have raised concerns about the effects that even temporary inundation would have on the wildlife and cultural heritage of the protected Special Areas around Lake Burragorang that sits behind the dam.

As The Sun-Herald has reported, a survey of just over a quarter of the area to be affected alone identified 334 cultural heritage sites. The heritage assessment report for the draft environmental impact statement (EIS), though, deemed impacts as likely to be “relatively minor”, prompting demands for a re-examination.

Mr Harwin declined to comment on the briefing note. Heritage NSW said it “continues to work closely with the Department of Planning” to finalise the EIS.

“As part of this process, modelling and assessment is under way to understand the impact on Aboriginal cultural heritage,” the Heritage spokesperson said.

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Kazan Brown, a Gundungurra traditional owner, said modelling and evaluation “should have been done from the start”.

“They have already made up their mind [to proceed with the project], so this is just ticking a box,” Ms Brown said.

She added that Indigenous elders were concerned about “the safety of any information” they shared, adding: “We don’t know what the information is going to be used for”.

Harry Burkitt, a campaign co-ordinator with the Colong Foundation for Wilderness, said Heritage NSW had done “an honourable job at resisting the political pressure from WaterNSW and the floodplain development lobby to destroy these UNESCO-listed Aboriginal cultural sites”.

Mr Burkitt also said Mr Bett’s presence in the co-ordinating group was notable because of his prior role as chief executive of Infrastructure NSW. The agency has supported the project because of the reduction on risks to residents in the Hawkesbury-Nepean floodplain below the dam.

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“Letting Jim Betts co-ordinate the Department of Planning’s review of the same assessments he directed at Infrastructure NSW is putting the fox in charge of the henhouse,” Mr Burkitt said.

A Planning spokesman said the meetings “provide early and effective engagement across government, assist applicants in ensuring all required documents are prepared to a high standard, and ensure any exhibited EIS fully addresses key issues of community interest, including biodiversity and Aboriginal cultural heritage”.

The spokesman added that Mr Betts was “not the decision-making authority” and the group had not met since September, its seventh gathering in 2020.

“Mr Betts stands by his comments that an EIS will conform with the requirements stipulated under legislation and will be exhibited and subject to full consultation,” the spokesman said.

A separate government spokeswoman said the updated Aboriginal Cultural Heritage Assessment will be shared with Registered Aboriginal Parties for review and comment. “Once finalised, the EIS will be exhibited publicly, which is expected to take place later this year,” she said.