A group of prominent scientists, former environment ministers and conservationists have written to the New South Wales premier condemning legislation to raise the Warragamba Dam wall.

The $670m plan to raise the dam wall by 14 metres was announced by the NSW government in 2016 as a strategy to prevent catastrophic flooding in outer-western Sydney.

But the plan is opposed by conservationists because raising the wall would flood 4,700 ha of the Blue Mountains world heritage area and “push several Australian threatened species towards extinction”.

Gundungurra traditional owners say raising the wall would also lead to the destruction of more than 50 recognised Aboriginal heritage sites.

On Tuesday a letter signed by a group of 20 prominent Australians, including former environment ministers Peter Garrett and Bob Debus and businessman Geoffrey Cousins called on the NSW premier, Gladys Berejiklian, to dump the plan.

They argue the plan would put Australia in “clear contravention” of the world heritage convention and would lead to the inundation of land which 25 threatened species are known or are likely to inhabit.

“These natural areas are of the highest conservation value in Australia that should be preserved at all costs,” the joint letter states.

“There is yet to be even an environmental assessment completed on the impacts of the proposed dam development. The largest of flood events would inundate up to 4,700 hectares of national parks and 65km of wilderness streams above the current full storage level of the dam.

“Inundation of this nature would result in extensive and irreversible damage to the integrity of the greater Blue Mountains world heritage area.”

But the NSW government says raising the dam wall is necessary to prevent “a catastrophic flood”.

“We believe it’s an appropriate trade-off to protect the hundreds of thousands of lives of people who live in the greater west of Sydney, particularly the 130,000 who live on the flood plain today,” the minister for western Sydney, Stuart Ayres, has previously said.

The 142-metre-high dam wall – Sydney’s main source of drinking water – was completed in 1960. It fences in Lake Burragorang, a 2,000 gigalitre lake on the eastern edge of the Blue Mountains.

It guards the Hawkesbury-Nepean Valley, which was identified by the Insurance Council of Australia as the most flood-prone area in NSW.

According to the Hawkesbury-Nepean Valley flood risk management strategy, which recommended the dam wall be raised, up to 134,000 people live and work on the floodplain. That number is growing as Sydney sprawls westward.

But associate professor Jamie Pittock from the Australian National University said there were other alternatives to raising the dam wall.

“The NSW government’s strategy for managing flood risk in the Hawkesbury-Nepean Valley is predicated on allowing more people to move into harm’s way,” he said.

“The assessment of flood control favours raising the Warragamba Dam wall by ignoring its environmental and social impact, and the benefits of any non-flood control alternatives.



“Management of the existing storage of Warragamba Dam, improvements to flood evacuation routes, increased flood forecasting capacity and adopting international best practice floodplain development controls are all alternatives to raising the dam wall.”

The NSW government introduced a bill into the state’s upper house earlier in September, ahead of an environmental impact statement expected by mid-next year.

If it goes ahead, construction is expected to be finished by 2024.