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)
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The chances of getting a warm year are increasing all the time. The chances of getting a cold year are decreasing all the time. When you look at shorter time frames, from day to months, it’s really about the chances of getting a very hot event, extreme high temperatures, or heavy rainfall, are what’s really impacting both ecosystems and human activities. It’s how extreme are changing that’s really important…As temperatures rise, the number of rain days decrease. So moderate rain decreases while extreme rain increases…more and more extreme rainfall events and floods, with concurrent impacts, including erosion and landslips. – Prof. James Renwick 2022
When Cyclone Gabrielle struck Hawkes Bay in January 2023 it provided some insight into the limitation of climate models. The 2020 NIWA report, Climate change projections and impacts for Tairāwhiti and Hawke’s Bay projected that under the RCP8.5 (worst-case) scenario the average maximum annual 5-day rainfall by the year 2081 would be 151.3 (+14.8)mm at Glengarry.
During the cyclone, the Glengarry site recorded 546mm of rainfall with almost 400mm falling in 12 hours at a maximum intensity of 56mm/hour.
Over New Zealand, an average two to three fold rise in frequencies of extremes occurs irrespective of seasons due to anthropogenic influence, with a mean temperature increase close to 1°C. – Thomas et al, October 2022
The ‘close to 1°C’ threshold across Aotearoa has now been exceeded:
NIWA’s long-running ‘seven-station’ series shows NZ’s average annual temperature has increased by about 1°C over the past 100 years. – Annual Climate Summary 2025, Earth Sciences New Zealand
This helps explain why extremes are increasing in magnitude and number. The average global temperatures 2023-2025 exceeded 1.5°C. Moreover:
Extreme weather events are rising at a pace which exceeds expectations based on thermodynamic arguments only, changing the way we perceive our climate system and climate change issues…. The additional evaporation and rainfall tends to end up in heavy rain rather than alleviating drought: Half of it comes down in the wettest 6 d(ays) each year. Even once global warming is stopped, we will see unprecedented extremes for a long time to come. Just think of a former once-in-5000-year event which at 1.5°C warming may have become a once-in-50 year event. – Di Capua and Rahmstorf 2023
The takeaway message is clear. Existing climate models and weather forecasts are underestimating the frequency and magnitude of extreme (acute) events. This should be considered when looking at the flood hazard mapping tool below, as the statistical probability of any AEP1% event may no longer be valid. In some locations, the probability of recurrence may increase to decadal (AEP10%) or even annual (AEP100%). That is, they occur every year.
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
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.
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).
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).
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.
See also:
Stationarity is a term used by river engineers, hydrologists, economists and insurance companies to work out the statistical likelihood of flooding based on how often it happened in the past, hence term such as 1-in-100-year-flood.