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Effects & Impacts: Floods – bigger and more often

The Rangitata River 2019 floods; the braided river reclaims its stolen braids. The B&W image is late 1960. Images: Canterbury Maps Historical Aerial Imagery CanterburyMaps & partners licensed for reuse CC BY 4.0 

From the ground, it would have seemed like chaos; floods of water rampaging over the plains, damaging anything in its path. But from above, a different picture was emerging. Environment Canterbury (ECan) staff were photographing the floods from the air, later stitching together the images to create a mosaic of the event. It showed the floodwaters were following a predetermined pattern. The flood was itself a river, with twists and braids and tributaries, much like the Rangitata itself. A zombie river, long ago buried beneath asphalt and housing and irrigators, had been revived. – The Rewilding Project / Stuff (2021)

  Impacts

Home > Climate wiki > Impacts > Canterbury flood risk

Floods: bigger & more often

Summary

More than 750,000 New Zealanders live in locations exposed to flooding from one-in-100-year rainfall flooding events. And this number could increase to more than 900,000 with a further 3 degrees of warming due to climate change…Around $235 billion worth of buildings across the country are exposed, which could rise to $288 billion if there is 3 degrees of additional warming; 26,800 kms of the nation’s roads, 14,100 kms of stormwater pipelines and 21% of national grid sites (e.g. substations) are also exposed to flooding under New Zealand’s current climate. This could rise to 30,800 kms, 15,400 kms and 29%, respectively, with 3 degrees of warming. – MfE & Stats NZ 2025

My fear is that this will turn into an insurance crisis, then a banking crisis into a government debt crisis. – Massey University banking and insurance expert Dr Micheal Naylor, 2024

You’ll often come across the statement that for every 1°C of warming, the atmosphere can hold about 7% more moisture. This figure comes from research undertaken by the French engineer Sadi Carnot and published 200 years ago this year. Yes, a hotter atmosphere has the capacity to hold more moisture. But the condensation of water vapour to make rain droplets releases heat. This, in turn, can fuel stronger convection in thunderstorms, which can then dump substantially more rain. This means that the intensity of extreme rainfall could increase by much more than 7% per degree of warming. What we’re seeing is that thunderstorms can likely dump about double or triple that rate – around 14–21% more rain for each degree of warming. – Dowdy et al, May 2024

  • There was a record amount of water vapour in the atmosphere in 2024 (Fig. 2).

It is not possible to ‘fix flooding’ and some level of flood risk would be present even if investment were significantly increased. There will always be a bigger flood event, or areas that cannot be practicably remedied. As described in the 8 September 2022 report that preceded this report:

‘Managing flooding is challenging in Christchurch as it is flat and low lying. Pipes, drains and waterways only have limited capacity so the city also relies on overland flow paths and flood ponding to deal with extreme events. We design our networks to direct stormwater and flooding towards parks and roads ahead of properties and homes. However, past practices have left a legacy of risk in some locations and there are still some very low lying buildings at high flood risk…’ – p181 Agenda, Christchurch City Council, 05 April 2023

  • Figure 1 is a screengrab of the Flood Hazards mapping tool showing the current and future flood exposure under a warming climate to people, property, and infrastructure. 

  Impacts

Summary

More than 750,000 New Zealanders live in locations exposed to flooding from one-in-100-year rainfall flooding events. And this number could increase to more than 900,000 with a further 3 degrees of warming due to climate change…Around $235 billion worth of buildings across the country are exposed, which could rise to $288 billion if there is 3 degrees of additional warming; 26,800 kms of the nation’s roads, 14,100 kms of stormwater pipelines and 21% of national grid sites (e.g. substations) are also exposed to flooding under New Zealand’s current climate. This could rise to 30,800 kms, 15,400 kms and 29%, respectively, with 3 degrees of warming. – NIWA 2025

My fear is that this will turn into an insurance crisis, then a banking crisis into a government debt crisis. – Massey University banking and insurance expert Dr Micheal Naylor, 2024

You’ll often come across the statement that for every 1°C of warming, the atmosphere can hold about 7% more moisture. This figure comes from research undertaken by the French engineer Sadi Carnot and published 200 years ago this year. Yes, a hotter atmosphere has the capacity to hold more moisture. But the condensation of water vapour to make rain droplets releases heat. This, in turn, can fuel stronger convection in thunderstorms, which can then dump substantially more rain. This means that the intensity of extreme rainfall could increase by much more than 7% per degree of warming. What we’re seeing is that thunderstorms can likely dump about double or triple that rate – around 14–21% more rain for each degree of warming. – Dowdy et al, May 2024

  • There was a record amount of water vapour in the atmosphere in 2024 (Fig. 2).

It is not possible to ‘fix flooding’ and some level of flood risk would be present even if investment were significantly increased. There will always be a bigger flood event, or areas that cannot be practicably remedied. As described in the 8 September 2022 report that preceded this report:

‘Managing flooding is challenging in Christchurch as it is flat and low lying. Pipes, drains and waterways only have limited capacity so the city also relies on overland flow paths and flood ponding to deal with extreme events. We design our networks to direct stormwater and flooding towards parks and roads ahead of properties and homes. However, past practices have left a legacy of risk in some locations and there are still some very low lying buildings at high flood risk…’ – p181 Agenda, Christchurch City Council, 05 April 2023

  • Figure 1 is a screengrab of the Flood Hazards mapping tool showing the current and future flood exposure under a warming climate to people, property, and infrastructure. 
Fig. 1.  Flood Hazard across Aotearoa New Zealand (1%AEP rainfall event). Click on the image to be taken to the website. Note: this does not take into account coastal flooding from a simultaneous 1%AEP storm tide event.
Fig. 1.  Flood Hazard across Aotearoa New Zealand (1%AEP rainfall event). Click on the image to be taken to the website. Note: this does not take into account coastal flooding from a simultaneous 1%AEP storm tide event.

Flood risks in a warming world

Flooding is New Zealand’s most frequent damaging natural hazard. Insurance claim statistics indicate damaging flood events have been increasing since the late 20th century. Future climate change will cause sea levels to rise and could increase heavy rainfall events potentially increasing flood inundation hazard. When coupled with urban development in or near active floodplains they would expose New Zealand to more frequent damage and disruption from flood hazard events leading to higher economic losses. – NIWA

The intensity of extreme rainfall could increase by much more than 7% per degree of warming. What we’re seeing is that thunderstorms can likely dump about double or triple that rate – around 14–21% more rain for each degree of warming. – Dowdy et al, May 2024

Fig. 2: The total amount of water in the atmosphere reached a record value in 2024, at 4.9% above the 1991–2020 average, markedly higher than in 2016 (3.4%) and 2023 (3.3%), the years with the second and third highest values. Image: Copernicus
Fig. 2: The total amount of water in the atmosphere reached a record value in 2024, at 4.9% above the 1991–2020 average, markedly higher than in 2016 (3.4%) and 2023 (3.3%), the years with the second and third highest values. Image: Copernicus

In 2024 the average global temperature exceeded 1.5°C above pre-Industrial levels. They declined to 1.44°C in 2025 but the trajectory is one-way. The oceans have also warmed much faster than models predicted. Together, this means that New Zealand will experience more frequent and higher intensity rainfall along the western coasts, particularly in the South Island. Rvers that originate in the foothills of the eastern side of both islands are likely to receive less rain, whereas the west will receive more. Click on the map (Fig. 3) to be taken to the website where you can explore different scenarios.

Fig. 3: By 2080, compared to the period 1986-2005, annual average rainfall is projected to increase 32.8% on the West Coast, with a similar reduction in rainfall across much of Canterbury (Image: MfE). However, the intensity of rainfall is likely to increase.
Fig. 3: By 2080, compared to the period 1986-2005, annual average rainfall is projected to increase 32.8% on the West Coast, with a similar reduction in rainfall across much of Canterbury (Image: MfE). However, the intensity of rainfall is likely to increase.

Nine of the ten most damaging floods in New Zealand between 2007 and 2017 occurred during AR [atmospheric river] events. – Reid et al. 2021

Under the RCP8.5 warming scenario, there will be a global doubling or more of the occurrence, integrated water vapor transport and precipitation associated with EAR (extreme atmospheric rivers), and a more concentrated tripling for the landfalling EARs, by the end of the 21st century. – Wang et al, 2023

Flooding from rivers that originate in the mountains will likely increase in intensity, although not necessarily in frequency. Intensity is often associated with numbers and duration of atmospheric rivers (Video 1). NIWA’s online tool estimates the magnitude and frequency of high intensity rainfall at any point in New Zealand: High Intensity Rainfall Design Systems (HIRDS).

 Video 1: Short explanation of atmospheric rivers.
Short periods of extreme rainfall may occur anywhere, resulting in an increased risk of pluvial flooding, with smaller foothill-fed rivers prone to flood between extended periods of drought.  

Drought dries out soils as well as rivers, making the ground less permeable, so that heavy rain following a drought can be more damaging as the water flows off the soil rather than being absorbed (Video 2).

Video 2 shows just how long it takes water to soak into parched ground, illustrating why heavy rainfall after a drought can be dangerous and might lead to flash floods. Video: Dr Rob Thompson, University of Reading

If you live in a flood-prone area, you can help by contributing photos to a national database to support understanding of flood hazard and flood risk. 

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