It took two weather systems to submerge much of western Sydney and drench the Mid North Coast in a metre of rain, to take the life of a young man in Cattai Creek, to wreck the bridges around Kempsey just repaired after the Black Summer fires, to leave more than 20,000 people evacuated.

On Saturday, March 20 a thick band of cloud developed off the north coast of Western Australia near the Northern Territory border and slowly began creeping across the continent, dumping rain as it went.

Two days earlier a weirdly long and stable low-pressure trough had developed off the east coast, extended from eastern Victoria. The two systems collided over NSW on Monday, intensifying showers that had begun days earlier and causing them to linger longer and intensify over a sodden landscape.

Long before the first drops fell the state’s emergency services machinery had begun to turn. Just as the Rural Fire Services manages our fire response when a state of emergency is declared, the State Emergency Service takes control for widespread flooding.

On Wednesday the SES established an incident centre for the floods and began deploying resources around the state - supplies and equipment, flood boats and helicopters. Volunteers from the ACT, Victoria and Queensland joined locals up and down the coast.

By Thursday afternoon, March 25, the SES had responded to over 12,000 calls for help and conducted 1022 rescues, deploying 1672 teams - typically of four people - in rotating shifts. Over 27,000 people had been evacuated. Soon the SES was dropping necessities to the stranded, typically food orders made to local stores by those with long experience of living in the lowlands.

“We bring in orders of things they really need, we don’t do beer and ciggies,” one SES officer helpfully explained.

Insurance Council of Australia chief executive Andrew Hall says that by Friday afternoon around 26,000 claims had been lodged, with an estimated total cost of over $385 million. This early figure was certain to rise. He noted that the $7 billion the industry had paid out in claims over the past three years was evidence of the intensifying impact of climate change.

“We dodged a bullet,” Minister for Western Sydney Stuart Ayres, told the Herald on Friday afternoon, grateful that the SES had performed so well, that the evacuation routes out of low lying parts of western Sydney held out as modelling had predicted, and that less rain than expected fell.

But it was a lot of rain. In parts of the north coast more than 1000mls were recorded, and more than 400mls fell over Western Sydney. Last Saturday Warragamba Dam began to spill, prompting evacuation orders.

Soon Sydney’s water desalination plant had to be switched from standby into production mode because the rain flooding into Warragamba after the fires had contaminated the water supply.

It was enough rain to reopen ongoing debates in Sydney over plans to raise the walls of the Warragamba Dam and accommodate tens of thousands more homes on the fertile floodplain of the Hawkesbury-Nepean Valley.

Bickering between Emergency Services Minister David Elliott and Water Minister Melinda Pavey flared at the start of the week over whether the government should have altered the operating rules for Warragamba Dam ahead of the deluge.

Elliott believed the water level should have been dropped ahead of the rain - as had been considered as policy as far back as February - so it could better cope with a sudden in-flow.

Pavey says that while the government had considered this, such a move would have required changing regulations governing the dam’s management. Because it is a water supply dam rather than a flood management dam, its contents can’t simply be emptied.

Ayres, a key advocate of the government’s plan to raise the dam’s wall by 14 metres to give it airspace to hold floodwater, says such a plan would have significantly mitigated the impact of this week’s flood.

Experts looking on from outside the government are less convinced.

Professor Robert Glasser, an expert in disaster risk reduction from the Australian Strategic Policy Institute, says Premier Gladys Berejiklian’s comment early in the week that the flood could not have been foreseen was simply wrong.

Global warming, he says, is here. Warmer ocean temperatures cause increased evaporation into a warmer atmosphere that can carry more moisture than is used. Until it can’t, and then it dumps rain and causes floods.

Glasser says the terms used by local and state governments such as “one-in-50-year” or “one-in-100-year” events no longer make much sense, because the models are no longer as accurate as they once were due to changing weather patterns.

All we know now is that we are going to have more fires, more droughts and more floods, he says.

Politicians, he says, know this too, and because public money is spent in rescue, recovery and rebuilding, they need to address this fact.

Professor James Pittock of the Australian National University is more blunt.

The Hawkesbury-Nepean Valley he says is “uniquely dangerous” due to what he calls “the bathtub effect” caused by inflows from across the catchment being caught up at the Sackville Gorge near Windsor, causing rapid water level increases across the floodplains in heavy downpours.

Current regulations prevent new development under the one in 100-year flood lines.

Ayres says that regulation coupled with evacuation infrastructure such as raised roads and good emergency response saved lives this week, and proved development in the valley could be done safely.

Pittock is unimpressed.

“Responsible risk management is asking yourself: What is the worst event that might occur, and how would you manage it?” he says.

“Stuart pissing around by saying, ‘Oh, we’re not developing on the floodplain because we’re not allowing new development below the one 100 year flood line’ is just awfully irresponsible.”

Eventually, he says, a one in 500-year flood will hit. When that happens, he believes, evacuees will move to those towns built historically on low hills such Penrith, Windsor and Richmond.

“As the floodwaters come up the escape routes would be cut, and you could end up with thousands of people stuck on those hills as the floodwaters rise to 20 metres.”

He believes the money slated to be spent on raising the dam wall - potentially $3 billion including environmental land offsets - would be better spent slowly buying out those people already living in the most flood-prone areas, and further development under the one in 500-year flood line banned.

Parts of the floodplain, he says, should be used for recreation and market gardens.

Nor does Pittock believe that either raising the Warragamba Dam wall, or changing regulations to allow its level to be drastically lowered before floods hit is sufficient to make the region safe.

“Obviously, if you’ve got empty air space in a dam, then that flood water that is immediately pulsing down river, and it may reduce the height of the flood water of the margins,” he says.

“That’s the most positive thing I can say about it.

“The reasons why I think it is a really stupid strategy is that the experience around the world is that when you build a big flood control dam it gives an excuse for developers and local governments and state governments to approve development on the floodplain by arguing that it’s safe.

“It is never, ever safe, and that’s because, no matter how big you build a dam, at some point there will be a catastrophic flood that overwhelms it.”

Ayres says that there is “absolutely, categorically” no way that a raised Warragamba Dam wall will have any impact on development. Its role, he says, would simply be to manage floods.

He says an extra 14 metres on the dam wall would have held around 1000 gigalitres of water, and if that water had been released later and more slowly, flood peaks down the valley would have been lower. If people really believe property should be resumed in parts of the valley they should cost such a proposal and bring it to local governments, Ayres says.

Glasser notes that budget pressure on local governments can prompt them to encourage development.

As the debate over flood and infrastructure went on John Brookes and his wife Sue were cleaning up their 200-year-old sandstone home in Windsor.

The lower floor had been inundated on Tuesday, but sandbags kept the sludge out and, he says, sandstone cleans-up well.

By Friday afternoon the yard had dried so well he ran the mower over it while inside he had ribbons dunked in eucalyptus oil blowing from fans to sort out the musty smell.

“We’re not going to leave the area, we love it,” he says. “We’ve been here all our lives.”

His first memory of the summer fires today is not the horror, but the sense of community drawing together.

“When the fires come you go and help out your mates who get hit, when the floods come you do the same. It is part of life around here and people know it.”

Besides, says Brookes, whose son is an environmental scientist, climate change is going to hit everyone, not just those who live in the valley.