The state government intends to raise the wall at Warragamba Dam up to 14 metres in an attempt to safeguard properties in the Hawkesbury-Nepean valley downstream from flooding during high-rain events, and released an Environmental Impact Statement (EIS) on the proposal for public review in late September.

The International Union for Conservation of Nature (IUCN) prepared a technical review of the EIS, specifically the section relating to the effects on the Greater Blue Mountains World Heritage Area, and determined it would be "inappropriate" to proceed with plans to raise the wall given the scope of the effect the upstream inundation would have on the heritage area could not be properly established.

The review stated the EIS World Heritage Assessment conclusion that the impacts the project would have on the world heritage area 'would not result in a material loss or degradation of the Outstanding Universal Value [OUV]' "is contradicted by the findings presented in the assessment itself".

"The upstream impact area for the raised dam clearly includes important cultural sites that contribute to the property's integrity," the review stated.

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"As outlined in the EIS, the project may result in the total loss of a number of known sites with high cultural and scientific significance as a result of their inundation.

"The inundation of these sites would, therefore, damage attributes of the OUV [Outstanding Universal Value] of the property, and therefore this reported loss appears clearly at odds with the conclusion of the EIS that the project 'would not result in material loss or degradation of the Outstanding Universal Value of the [heritage area]'."

The review notes the "cultural context" of the heritage area could not be separated from its intrinsic value.

"The consideration of cultural associations relevant to OUV is clearly not rigorous in the EIS," the review states.

"There have been no physical investigations to enable informed assessment of the sites concerned, and the approach to understanding cultural values requires broadening to encompass concepts of place, landscape, contemporary tradition and living heritage, rather than limiting cultural heritage to known individual sites."

Also highlighted was the insufficient field surveys completed in bushfire-affected areas following the massive blazes of 2019/20 and the plan to offset impacts of the project (as the OUV "cannot be subject to excisions and compensation on an area basis" and "the concept of compensation plots for the planned loss of OUV is not appropriate").

The review concludes that the wall-raising project, as currently proposed, "would directly degrade OUV... [and] IUCN considers that proceeding further with the implementation of the project appears to be inappropriate in relation to the requirements of the World Heritage Convention".

Bob Debus, former NSW environment minister and chair of the Colong Foundation for Wilderness (which has strongly opposed the wall-raising proposal from the start), said proceeding with plan was dangerous.

"The UNESCO advice is saying quite clearly that raising of the Warragamba Dam wall is totally inconsistent with Australia's obligations under the World Heritage Convention," he said.

"Despite the torrent of expert condemnation from UNESCO, [Western Sydney minster Stuart] Ayres merely dismisses these reports as a flesh wound. He has become the Black Knight of Warragamba Dam.

"Stuart Ayres is putting Australia on a collision course with the World Heritage Committee, a situation that can only end with an in-danger listing for the Blue Mountains."

A representative from Mr Ayres' office told the Advertiser last month the minister believes the plan to raise the dam wall "should go ahead."

"A decision on the proposal will only be made after every 'i' is dotted and every 't' is crossed," the representative said.

Indigenous residents, scientists, environmental action groups, local councils, politicians and UNESCO have all raised various concerns about the plan.