“To introduce legislation that will allow the destruction of Sydney’s iconic wild rivers before the [EIS] and community consultation for the project is even completed shows this government has no respect for the community or nature," he said.

Avoid a 'catastrophic flood'

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However, Stuart Ayres, minister for western Sydney, said raising the wall would expose only 0.06 per cent of the World Heritage area to inundation and would help avoid "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," Mr Ayres said.

The Hawkesbury-Nepean region downstream of the dam is recognised as a high-risk flood zone given its unusual confluence of rivers with a narrow exit to the sea. The potential is there for a "bathtub" effect that is slow to drain during big rain events.

Major floods have dogged the region over the two centuries of European settlement although it has not stopped planners allowing major housing and other infrastructure to be built on the plain.

Paul Broad, formerly chief executive of InfrastructureNSW, is a strong supporter of the wall-raising project.

"It's the best thing that could be done for infrastructure in this city by miles," Mr Broad told the Herald.

While it's too late to reverse inappropriate development in the region, it's also not right to play "Russian roulette" with floodplain residents who might struggle to escape a major flood, he said.

'Grossly inadequate'

Opponents, though, query the government's haste, not least because Warragamba Dam is about two-thirds full. Water levels are falling amid the deepening drought, with Sydney's storages lately losing about 0.7 per cent a week, WaterNSW data show.

Chris Minns, Labor's water spokesman, said Labor would oppose "reckless legislation that will cause untold damage to this priceless piece of Australia’s natural heritage".

“The NSW government has done next to nothing to examine enhanced evacuation routes or come up with a detailed emergency management plan in the event of a catastrophic flood in their seven and a half years in office," he said.

Evidence suggesting the process to secure project approval has been rushed includes an email exchange between a Gundungurra traditional owner Kazan Brown and a consultant hired by government to survey Indigenous sites that face inundation if the wall was raised.

The survey had 25 days, a period the consultant said was "grossly inadequate for the size of the project", according to an email obtained by the Herald.

"After much negotiation they have still not given any more days. So, we will survey for two weeks and then present them with [what] we have found and see where we go from there," the consultant Sam Richards, said in the email.

Mr Richards declined to comment when contacted by the Herald.

A spokeswoman for Infrastructure NSW said the assessment of Aboriginal culture heritage was "integral" to the impact study.

"While 25 days was an initial guide, surveys involving the Registered Aboriginal Parties are due to recommence this month," the spokeswoman said.

"The survey effort is ongoing, and is being undertaken in line with the methodology that has been agreed with the Registered Aboriginal Parties, not based on a specified number of days in the field," she said. "It will take as long as required to satisfy the agreed methodology."

Harry Burkitt, a campaign manager with the Colong Foundation for Wilderness, described the amendment bill as "an existential attack on the National Parks and Wildlife Act, World Heritage, the Blue Mountains, wild rivers and wilderness".

“Not since the Franklin Dam proposal have we we seen such a blatant disregard for a World Heritage site in Australia," he said.