Longer, more frequent heatwaves in the country's largest city could harm and even kill people if more is not done to prepare, heat researchers say.

Auckland Council has published research showing the number of hot days over 25C could double by 2050, under a moderate climate change scenario.

Further into the future, or under a more severe climate scenario, the number could increase by as much as five-fold.

The research also showed that heatwaves lasting weeks are a possibility in future - with often no reprieve at night-time either.

Council principal sustainability and resilience adviser Lauren Simpson said at the moment, the longest heatwave during a hot summer was about four days.

"That will shift to the longest heat wave being about 20 days in 2090 under a high warming scenario, so really extended periods of hot weather," she said.

"With that will come health impacts. That is compounded when you have consecutive hot nights as well, because long periods of hot days and hot nights mean that the body doesn't have time or doesn't have a reprieve from the heat."

The new paper is a follow-on from previous research the council published in 2024, showing that Auckland had an urban heat island effect - where the CBD and other highly urban areas were several degrees hotter than the rest of the city.

The latest research said that effect would compound future heat.

"Urban areas such as city centre show the greatest number of night-time heat events in summer, with the highest minimum temperatures. These areas are at greatest risk of adverse health impacts."

The scenarios ranged from a low emissions pathway (SSP1) to a high emissions pathway (SSP3), using Auckland's hottest summer to date - 2021 to 2022 - as a baseline.

"That wasn't an average summer - we're trying to look at the worst-case scenario," Simpson said.

The research did not use the most extreme future scenario (SSP5), which the government has criticised councils for including in their planning.

Recent heatwaves in Europe and the UK have been blamed for thousands of excess deaths, and have fanned wildfires across the continent.

Simpson said heatwaves in New Zealand were still uncommon, but the research showed the need to prepare now.

"We haven't yet experienced the kind of heat that we're seeing in those other cities, and so they're providing a really great opportunity to learn from, to see what works, because lots of those cities, they're learning themselves."

Ahead of this summer and a strengthening 'super El Niño', the council was planning a campaign with advice for how to keep cool, she said.

"As part of that, we're developing a cool spaces map that will help people locate cool spaces in their local communities and that might be shading parks or public facilities with air conditioning, local beaches, bottle refill stations, that sort of thing."

Heatwaves compound health effects - researcher

Otago University professor Simon Hales, who has researched extreme heat for decades, said people could experience its effects after one day.

"But the effects on health are certainly greater if the heatwave is more prolonged - if there's no relief at night and if it continues for several days," he said.

That was likely because the physiological stress of extreme heat, as well as the environmental effects, slowly built up.

"People are getting maybe dehydrated as well as too hot - their bodies are not managing to keep up with the cooling, and our houses and infrastructure are gradually heating up more and more as the heatwave continues."

Research Hales collaborated on in 2000 found that for every degree of heat over 20C, the mortality rate in Christchurch increased by one percent.

The methods in that research were cruder than subsequent research, but the link between extreme heat and excess deaths and hospitalisations had since been clearly established across hundreds, "maybe even thousands" of scientific papers, he said.

That included research in 2024 that found more preschoolers ended up in New Zealand hospitals on hot days.

Children, older people and those with existing health conditions were all vulnerable to heat, he said.

"Many of the main chronic health issues - things like cardiovascular disease, respiratory disease, psychiatric problems - many of these things are sensitive to heat in the short term, so you see increases in hospital admissions and deaths from those things."

Heatwaves are defined as several consecutive days where temperatures fall in the top few percent of what is considered normal for that time and place.

"It's a little bit hard to understand at first glance," Hales said.

"If you take the example of a city that's in the north of Europe, people might start to have an increase in the health effects of heat at temperatures that we would consider to be quite comfortable," he said.

"Likewise, people who live in tropical countries probably would not have any effect on their health at temperatures that we would start to be thinking, 'this is getting pretty hot'."

The preparation for future hot weather in New Zealand was "lagging a bit, but it's not disastrous".

"The more important question is, how's our preparation for [mitigating] climate change at the moment? And I would give you a very different answer to that question - I think, as a country, we are going backwards."

Green space, building design provide possible solutions

Simpson said creating more green space in urban areas could both mitigate climate change and help cities adapt to it.

"Not only do trees and plants help to cool our urban environment, but they provide a myriad of other benefits - from supporting our biodiversity, stormwater management, improving the aesthetics and connection for people with nature, and absorbing or sequestering carbon."

There were additional things to consider in urban planning and development, such as using building and paving materials that reflected heat instead of absorbing it.

That included thinking about how homes were built, she said.

"In recent times we've made quite a good shift to making our homes warmer and drier. We now need to be thinking about making them cooler."

Last year, Auckland Council and the New Zealand Green Building Council (NZGBC) sent a joint letter to the government asking for changes to the Building Code, requiring new houses to pass a check at the design stage to ensure they would not overheat.

NZGBC chief executive Andrew Eagles said half of new builds now were terrace houses and apartments, and many of them were overheating too frequently.

"We don't recess our windows, so you're not shading that space. And at the same time we've had a love affair with very large glazing - and that so the combination of large glazing and no cross-ventilation and not recessing means the house just can't get rid of all that heat."

Many designs would require only minor tweaks, such as slightly smaller windows - which would save money too - and larger eaves, he said.

"It's all things that can be manageable, but if you don't have to do that check then the home is going to overheat for large parts of January, February, March, causing really significant discomfort."

With the right design, it was possible to have a home that was both warm in winter and cool in summer, Eagles said.

"We need to destroy a myth that it's one or the other."