The IUCN downgrade, one of four declared last week for Australian World Heritage sites including the Great Barrier Reef, followed "devastating" blazes that affected 71 per cent of the Blue Mountains.

The region is now rated as being of "significant concern", down from "good with some concerns" in the IUCN's two previous reports in 2014 and 2017.

It is home to "primitive species of outstanding significance to the evolution of the earth’s plant life" such the Wollemi Pine, 90 per cent of all eucalypt species, and many threatened plants and animals, the group said in its report. The full fire toll is still being assessed to determine how threatened species were affected.

Climate change is considered as a "very high threat" for future fires, with the projected temperature increase over the 75-year period likely to be "beyond the adaptive capacity of most vertebrates".

Among the IUCN's list of potential "high threats" to the region's so-called outstanding universal values are plans by the state government to raise the height of the Warragamba Dam by 17 metres to reduce the flood risks in the Hawkesbury-Nepean flood plain.

Lifting the height of the dam will increase the frequency, duration, depth and extent of temporary
inundation upstream of the wall, affecting as much as 550 hectares, the IUCN said. Wildlife, wilderness and Indigenous cultural values will be some of the key features hit.

A report prepared from a draft EIS details the potential effects of the billion-dollar project on the region's World Heritage values.

Prepared by SMEC consultants before last summer's bushfires, the report says: "There is a lack of knowledge about the impacts to Blue Mountains plant species and vegetation communities from temporary inundation, and also the presence of threatened species in the potentially
impacted area."

But they conclude that "while there may be loss of some biodiversity, this would not significantly impact the [World Heritage Area] as a whole," with only "minor" impacts on Aboriginal cultural heritage.

Harry Burkitt, a campaigner with the Colong Foundation for Wilderness, said the IUCN assessment underestimated the area likely to be affected by inundation by at least a factor of 10, putting the at-risk area at 6000 hectares.

"IUCN's World Heritage Outlook is a strategic overview that has used preliminary data in determining its findings," Mr Burkitt said.

He also said the government's consultants, SMEC Engineering, were "being paid millions of dollars to systematically justify the destruction of the Greater Blue Mountains World Heritage Area and its hundreds of threatened Australian species".

"We know from botanical experts that combined with last summer's devastating bushfires, raising Warragamba Dam would put the very species constituting the Blue Mountains' Outstanding Universal Values at direct risk of extinction," he said.

"These iconic species include the Regent honeyeater, the Camden white gum, and Kowmung hakea."

By Peter Hannam