It’s worth noting that the government has looked at alternative methods of flood adaptation in the Hawkesbury-Nepean region and found that raising the dam wall is the best option in terms of minimising risk to life, property damage and cost. Critics of the project, however, argue that the environmental offsets needed will push the project’s price tag well above the $1.6 billion mark.

One of the biggest supporters of raising the dam is Minister for Western Sydney Stuart Ayres, who said lifting the wall, which formed just one part of a range of other flood mitigation strategies, was “the single biggest piece of work we can do to save lives and property, and it is also the most cost-effective solution”.

“Raising the dam wall will not allow any additional development than what is already permissible and will not see a reduction or lowering of the flood-planning level,” he said.

Meanwhile, Labor leader Chris Minns has indicated he would ditch the project in favour of lowering the maximum capacity of water in the dam, allowing the flood mitigation zone to be included in the existing height of the dam. He told 2GB last week he would prefer to update existing infrastructure to improve evacuation routes, as well as increasing the number of levees.

The United Nations Educational, Scientific and Cultural Organisation (UNESCO), Indigenous communities and environmental groups are also against increasing the size of Warragamba Dam because it would mean inundating a portion of the Blue Mountains World Heritage Area that sits around the dam’s huge Lake Burragorang reservoir.

Professor of civil and environmental engineering at the University of New South Wales Stuart Khan said it was possible for a water supply dam to also carry out flood mitigation, and suggested lowering the maximum water capacity was the best approach. However, Khan said this would decrease the amount of drinking water the dam could hold and mean Sydney would need to look at alternative water sources to improve water security.

The levee paradox

Former Deputy Commissioner of the NSW State Emergency Service Dr Chas Keys said while raising the dam wall could lower the risk of spilling, it would generate complacency in communities downstream that they would be protected, which would outweigh any positives it could offer. This is known as the levee paradox. Instead, he said all development in flood plains should be halted and a merit-based approach that would evaluate flood records and only approve those above the worst flood on record should be built. He added that a buyback system needed to be made compulsory.

Keys said he hoped his approach would form part of the NSW flood inquiry report, due to be handed down in coming weeks by Professor Mary O’Kane and former police commissioner Michael Fuller. Keys also wants the final report to scrap any plan of raising the dam wall.

It comes as the projected populations in flood-prone areas in Sydney are expected to skyrocket. Areas in the north-west of Sydney have a target of 33,000 new homes to be built before 2026 across Blacktown, The Hills Shire and Hawkesbury councils, while the south-west councils of Camden, Campbelltown and Liverpool have a similar target.

The issue of whether residents should rebuild in flood-prone areas has become increasingly contentious in recent years, with the government indicating it would conduct a review into the matter this year.

The Department of Planning and Environment said in a statement this inquiry would also examine “appropriate action to mitigate future flood risks to communities and ecosystems and the current and future land use planning, management and building standards in flood-prone locations across NSW”.

“The department will be guided by the outcomes of the inquiry ... due to be handed down at the end of this month.”

Chief executive of the Insurance Council of Australia, Andrew Hall, said the pressures of population growth in the Sydney basin, coupled with historical poor land use planning decisions, meant there was no simple solution when it came to insurance.

“The reality is the money that we would spend on raising the Warragamba Dam wall, potentially after a considerable period of further studies and complicated environmental approvals, could be spent immediately on a range of other mitigation measures in the most vulnerable areas of western Sydney,” he said. “If a decision is made to raise the dam wall, further development on the floodplain must be avoided.”

Hall added that climate change would only continue to drive the frequency and severity of extreme weather events, which would have implications for the affordability and availability of insurance in Australia. The council has previously noted that in some Hawkesbury Nepean locations with extreme flood exposures, insurance with flood cover included can be four or five times more expensive than the national median.

While the state and federal governments ponder whether to raise the dam wall, NSW Premier Dominic Perrottet said this week it’s time to rethink how we plan for long-term development, communities and the economy.

“We need to make sure that we can stand here as leaders in a circumstance where an event like this happens in the future – which it will inevitably will – that we have done everything we can to make sure we don’t make the mistakes of the past,” he said.