The NSW government’s decision to revive development on the Hawkesbury floodplain ignores long-established flood risks, evacuation limits and the growing impact of climate change.

It goes on and on. Governments in New South Wales seem unable to resist developing new suburbs in the valley of the Hawkesbury River on Sydney’s north-western fringe. Huge flood disasters are still being courted.

This time it’s an ALP government, but it has been the Coalition in the past. The plan was for a 6000-dwelling development at Marsden Park North which would have housed close to 20,000 people, but ministers Paul Scully (Planning) and Pru Car (Deputy Premier) have announced that it will be scaled back to 960 dwellings for about 3000 residents. Marsden Park North is on South Creek, a tributary of the Hawkesbury which enters the main stem of the river just below Windsor and up which huge backwater flows can occur during floods. A big Hawkesbury flood would create the largest and most complex flood evacuation one could imagine in NSW, and a failure to bring it off might lead to a death toll of epic proportions.

The problem of Hawkesbury floods is far from new. The valley has always been a magnet for development and its floods have been problematic since the first decade of settlement there in the 1790s. Governor Lachlan Macquarie sought to discipline farmer settlement in the Windsor area between 1810 and 1820, with little success, and since World War II suburban growth in the wider valley on Sydney’s fringe has always been strong. Periodically, people living there have been assailed by floods, most severely in 1867 when Windsor itself was almost submerged.

The last state Liberal government sought to give housing in the valley a measure of protection by raising Warragamba Dam as a flood mitigation measure: floodwater would have been stored above the dam and the flood peak below it lessened slightly as a result. But the environmental damage above the dam would have been colossal and Labor, returning to power in 2023, scotched the idea and with it the notion that further suburban development would have been made ‘safer’ and thus facilitated. The new government also knocked back the Marsden Park North development with the line “No more building on high-risk floodplains”.

Then came the problem of Sydney dwelling approvals falling below the level needed to match demand, and in response the government has back-flipped on its 2023 decision. Marsden Park North is on again, albeit in scaled-down form. A dreadfully dangerous development is now merely extraordinarily bad.

The problems here are many. Marsden Park North on rare occasions will be flooded massively. More houses and more people will add to the Hawkesbury problem of the population needing to evacuate when big floods develop, and there are known gaps in funding for transport infrastructure. An evacuation operation will be complex, with perhaps as many as 100,000 people valley-wide needing to evacuate in a worst-case scenario. The pressure on the population, and on the emergency services, will be great.

As always, some people will refuse to leave their homes. There will be severe congestion on the roads out of the valley and it is possible that people will be caught on shrinking islands or stuck in long queues in their cars. They will be overcome by rising floodwaters.

NSW has long been a leader in floodplain management in Australia. In the 1950s it developed the first large-scale government-led levee building projects, and it pioneered the land use controls which eventually saw the floor levels of new houses required to be above the level of the 1 per cent Annual Exceedance Probability (AEP) (so-called 100-year) flood. On both scores NSW was well ahead of Queensland, the two states sharing roughly equally 80 per cent of the national flood problem as measured by financial costs.

The present government is endangering the state’s floodplain management legacy. It also risks making insurance cover against the flood threat – already very costly in the state’s many known flood-liable areas – even more so.

In Australia, nearly a quarter of a million dwellings are thought to be exposed to severe to extreme flood risks. It is surely unwise to add to that number. The NSW government is consciously doing just that.

And then there is climate change. It has become increasingly clear that warming, by increasing the capacity of the atmosphere to hold moisture, will increase the severity of floods when the release of that moisture in the form of intense rainfall is triggered. What can be expected is floods higher than they would otherwise have been, and very big floods occurring more often.

The Hawkesbury has the potential for phenomenal flood heights thanks to the unusual geography of the valley. The river rises in mountains, flows through floodplains from upstream of Penrith to below Windsor and then enters the hilly country of the Sackville Gorge. Many tributaries deliver water to the river during floods (including the Nepean, the Grose, South Creek and Eastern Creek), but drainage is impeded by the gorge. Thus potential rises in flood levels above the gorge are greater than in any other river valley in the state. The so-called Probable Maximum Flood (the worst case thought possible) at Windsor is nearly nine metres above the assessed level of the 1 per cent AEP event there and seven metres above what was recorded in 1867.

These figures should concern us. A higher standard is needed in the Hawkesbury than elsewhere in the state. “No more building on high-risk floodplains” was a good idea in 2023, and it still is. Housing growth may be falling short, but putting more people at risk of big floods is no way to overcome that problem. The government is adopting an appallingly easy way out of it.

Whatever happened to the notion that the first duty of government is to keep people safe?