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He said if routes out of the flood-prone areas are not built, extreme floods could cause hundreds of deaths as people who had been cut off retreat to islands before being washed away.

Second, he said the government should halt plans to build more housing on the floodplain below the Warragamba Dam.

“We shouldn’t be considering doubling the population of the Hawkesbury-Nepean valley to 250,000 people over the next 30 years,” he said.

“I know it is a growing metropolitan areas and they’ve got to go somewhere. But I don’t think they have to go into an area that’s got this serious and well known problem.

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He said we have come to see rivers as just the channel in which they normally flow, but in fact floodplains are part of river systems too, and rivers will “reclaim them from time to time”.

“Floods are rivers’ way of reminding slow learners that they own the valleys,” he said.

Professor Stuart Khan from the University of NSW Global Water Institute agreed with Dr Keys’ concern about the amount of water flowing into the catchment below the Warragamba Dam.

Further, he said it appeared that the amount of water expected to flow into the system above the dam - estimated to be about 1500 gigalitres - would eventually overwhelm the dam’s capacity even if it had been raised.

The result, he said, would likely be a delayed and perhaps even a slightly shorter flood peak.

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“I don’t know that that would be a great deal of comfort to people who are flooded, though.”

Stuart Ayres, the Minister for Western Sydney, has previously told the Herald a higher wall would “create a 14-metre flood mitigation zone [that] would reduce average annual flood damages by about 75 per cent”.

The government has been approached for comment.

Bob Debus, the former Emergency Services Minister and chair of the Colong Foundation for Wilderness, which opposes raising the dam wall, said that it was dangerously misleading to suggest the project could stop floods.

“It is very important to remember that it is not possible for Warragamba Dam, whatever its size, to stop the most extreme floods. Leaked charts published in the Sydney Morning Herald have shown that raising the wall would have only a moderate effect even in a one-in-500-year flood event.

“I take it to be very significant that the Australian insurance industry has now withdrawn their support for the dam wall raising as the critical element in flood management and mitigation in the Hawkesbury-Nepean Valley.”