Construction of more than 10,000 homes in high flood risk areas across Western Sydney is set to be blocked.

The NSW Government has announced that it has abandoned proposals for two massive housing projects across the flood-prone Hawkesbury-Nepean Valley in Sydney’s west.

Meanwhile, plans for a third development have been scaled back.

Specifically, the government announced that:

• rezoning proposals and draft plans for a new Marsden Park North precinct and Riverstone Town Centre will not proceed; and
• plans for a new precinct at West Schofields will be scaled back and subject to conditions.

As a result, only 2,300 of the 12,7000 homes which have been proposed across the three developments will now proceed.

The Government will work with councils and other stakeholders to explore suitable alternative land-use options in the area.

Work is also underway to identify potential areas where additional housing may be accommodated in order to mitigate the impact of the decision on the pipeline of new housing development.

The announcement came as the Government also released new modelling for flood evacuations in the Hawkesbury-Nepean Valley.

The modelling was undertaken to help inform decisions on emergency evacuations, land use planning and road upgrades across the region.

(map of the Hawksebury-Nepean Valley floodplain. Image source: NSW Government https://www.nsw.gov.au/sites/default/files/2023-08/Flood%20Evacuation%20Modelling%20Report%20May%202023.pdf)

The latest developments come amid growing concerns about the level of flood risk associated with ongoing development of the Hawkesbury-Nepean Valley.

Covering an area of more than 500 square kilometres of floodplain in Western Sydney, the Valley is considered to be the most significant area of unmitigated flood risk in Australia on account of its unique landscape and its existing population.

In terms of the landscape, two features account for the region’s particularly high-risk profile.

First, the valley comprises of three main floodplains – Wallacia; Penrith/ Emu Plains; and Richmond/Windsor (including backwater flooding in South Creek and Eastern Creek).

Each of these floodplains are highly interconnected, with floods events occurring almost simultaneously across the floodplains in the valley.

Beyond that, a unique ‘bathtub’ effect created by the area’s geology means that floodwaters across the floodplains in the Valley can be deeper and more extensive compared with those in most other floodplains across New South Wales and Australia.

Whereas most river valleys tend to widen as they approach the sea, the opposite is the case in the Hawkesbury-Nepean. Whilst upstream catchments flow into the Hawkesbury Nepean River, natural choke points downstream are created by narrow sandstone gorges downstream between Sackville and Brooklyn.

As a result, floodwaters back up and rise rapidly. This can lead to deep and widespread flooding across the floodplain.

According to the aforementioned modelling, the effect of this is ‘like a bathtub with multiple taps turned on, but only one plug hole to let the water out’.

Not surprisingly, the impacts can be particularly severe in low-lying areas, which may become surrounded by floodwaters during a flood event.

As floodwaters rise, these areas become isolated when low lying roads are cut, creating flood islands. These may become fully submerged as the waters continue to rise. (Many of the urban centres including Windsor, Richmond, Pitt Town and McGraths Hill are located on these flood islands).

Additionally, major flooding events can result in a widespread ‘inland sea’, which can span many kilometres across.

Under storm conditions, this can lead to waves of up to 1-2 meters in height.

As a result of all this, staying and defending properties may be unsafe (rescues will also be perilous). In such cases, the only safe option for residents or workers is to try to escape before roads are cut off.

All this is happening as the region’s population has grown.

All up, the Valley caters for more than 140,000 residents and workers and is home to more than 36,700 residential properties in the floodplain along with 4.5 million square kilometres of commercial space.

As a result of all this, the modelling suggests that there is already a significant risk to loss of life during floods.

Even without any further rezonings, it suggests that in light of development which is already committed, as many as 980 people will be at risk of being unable to evacuate throughout the Valley in a one in 500-year flood event by 2041 (refer fig 4.2 on p53 in link above).

Were the three aforementioned developments to proceed, this number would increase to 1,200.

(More broadly, the modelling suggests that if ALL potential development options in the valley were to proceed, as many as 23,700 people would be at risk of being unable to evacuate during a one in 500-year flood. This includes not just the three aforementioned areas but also development options in Hawkesbury, Penrith City Centre (stage 2 and 3) and Penrith Lakes.)

NSW Deputy Premier Pue Car said that the new modelling would help to ensure that development across Western Sydney was restricted to suitable areas.

“Western Sydney residents have borne the brunt of recent disasters including the pandemic and floods in the Hawkesbury Nepean Valley,” Car said.

“By stopping unsafe development in dangerous areas on flood plains, and with our Government’s work to reduce the risk of disasters before they

(the ‘bathtub’ effect of the floodplains in the Hawkesbury-Nepean Valley Image source: Hawkesbury-Nepean Valley flood evacuation modelling to inform flood risk management planning, NSW Government October 2023)

happen, we’re making sure communities across Western Sydney, in areas including Penrith, Blacktown and Riverstone, are finally supported and better protected.

“When we consider new housing areas, we will look at both the potential for those homes to be inundated in floods, as well as the impact more homes will have on the ability of both new and existing residents to evacuate in emergencies.

“We know we can’t stop natural disasters from occurring, but we are committed to doing more to prepare for and prevent the worst of their impacts.”

“This new tool will not only help us better plan for evacuations but it will also make our amazing emergency service workers safer by reducing the risks they face when responding to floods in the valley.”

Insurance Council of Australia CEO Andrew Hall welcomed the decision.

Hall noted that this was the first meaningful decision which has been taken by a state government in response to a 2022 National Cabinet agreement to end the practice of developing on floodplains.

Noting that some residents of Western Sydney have experience flooding four times in the past three years, Hall said the decision will help to protect families and businesses from dangerous future flood events.

“The Minns Government should be commended for taking such a critical decision which will put the safety and wellbeing of future Western Sydney communities first above all other demands,” Hall said.

“This is a significant shift in thinking about how we make the region safer and improve its risk profile.

“Insurers appreciate that Sydney is grappling with a housing crisis as the population surges, but repeating mistakes of the past through poor planning decisions would only condemn future generations to trauma and financial loss through devastating flood events while placing further pressure on all insurance premiums.

“Western Sydney already is one of Australia’s most exposed and high-risk flood areas, due to the topography of the Hawkesbury Nepean basin.

“To have allowed developments to proceed in these areas in full knowledge of the flood risk would have been unforgiveable.

“It was never a question of whether these areas may flood; the science, data and modelling show we know they will flood – to put further housing in these areas of unmitigated flood risk would have been a terrible strategy.”

However, Tom Forrest, CEO of developer lobby group Urban Taskforce Australia, cautiones that a ‘Plan B’ will be needed for new housing following the decision.

With NSW needing to deliver as many as 376,000 new homes over the five years from July 2024, Forrest said that development will be needed across both infill and greenfield housing.

Meanwhile, Westerns Sydney will also require a diversity of housing to promote jobs, economic growth and housing choice.

In particular, Forrest would like to see greater planning flexibility in those areas which are designated as ‘Metropolitan Rural Areas’ which are not located in flood prone areas.

He would also like the government to review the impact of a proposes reintroduction of new water infrastructure charges – charges which he says are adding significant cost to new greenfield development in Sydney’s west.