Western Sydney’s Hawkesbury-Nepean Valley faces a serious, ongoing flood risk that has no simple or single solution, according to the first stage of a NSW Office of Water review.

“Infrastructure options can reduce but not eliminate the risk to life and property,” the report on Australia’s biggest flood risk says.

“Effective evacuation is the only measure that can guarantee to reduce the risk to life.”

The report says raising the Warragamba Dam by 15-23 metres is the most effective infrastructure option, but this needs further cost-benefit analysis.

A higher dam crest would reduce the frequency of large events and extend evacuation times, but it would not eliminate extreme flood risk.

Changes to the operation of dam gates and a five-metre cut to the full supply level should also be considered. These measures could be introduced sooner but would affect only minor to moderate floods, the report says.

“Significantly less cost-effective than raising the crest of Warragamba Dam wall are infrastructure options to enhance drainage of floodwater from the valley.”

The review finds limited potential for the use of levees, due to extreme flood depths, particularly in the Richmond-Windsor area.

Other recommendations focus on the evacuation capacity of transport infrastructure, ensuring integrated governance, improving flood data and modelling, recovery planning and community education and information accessibility.

The report calls for better policies and guidelines on land-use planning as population in the area increases.

The NSW Government has established a taskforce to lead the next stage of the review, which will further examine the recommendations.

Several weeks ago Suncorp CEO Personal Insurance and Insurance Council of Australia President Mark Milliner expressed concern about the continued building of houses in the dam flood area.

“There are 43,000-plus properties at the base of the dam that would be inundated if the dam failed – and yet we build more,” he said. “I am concerned when I hear local governments say development in high-flood-risk areas is ‘inevitable’.”

The NSW Government report says the Hawkesbury-Nepean valley has the potential to “fill like a bath”, given its large upstream catchments and narrow sandstone gorges that create natural choke points.

But previous surveys have shown residents often underestimate the risks.

Many evacuation routes feature low points that can be cut off before higher areas are inundated, potentially leading to isolated flood islands that could be submerged in extreme events, putting lives at risk.

“Large flood events could affect the entire regional and NSW economy by affecting transportation routes and utilities outside the flooded area,” the report says.

River levels reached 19.7 metres above mean sea level at Windsor during the valley’s record flood in 1867.

The report says a similar event now would cause about $4 billion in damage and require evacuation of about 45,000 people.

In a “worst possible” flood about 73,000 people would need to be evacuated and more than 20,000 homes would be at risk.