Heat (Extreme)

There is no standard definition, however the World Meteorological Organization (2015) defines a heatwave as:

"a marked unusual hot weather (max, min and daily average) over a region persisting at least two consecutive days during the hot period of the year based on local climatological conditions, with thermal conditions recorded above given thresholds."

The impact of heat on health

Exposure to hotter than average conditions compromises the body’s ability to regulate temperature and result in rapid rises in the human body gaining heat. Even small differences from seasonal average temperatures are associated with increased illness and death and excessive heat can occur during extreme hot spells in warm and cool countries alike (Vicedo-Cabrera et al. 2021).

The duration of excessive heat plays an important role in how people are affected by a heat wave. Extreme heat can cause illness and death and can affect anyone at any time. For example; heat rash, cramps, sunburn, heat exhaustion, heatstroke/sunstroke. Temperature extremes can also worsen chronic conditions, including cardiovascular, respiratory, and cerebrovascular disease and diabetes-related conditions.

Although anyone at any time can suffer from heat-related illness, some people are at greater risk than others. Babies and infants, older people, those with pre-existing medical conditions or on certain medications, and people living alone are more at risk. Pets and livestock are also susceptible.

  • Stay out of the sun, avoid extreme physical exertion and drink lots of water.
  • Put sun safety practices into action.
  • Wear loose, cool clothing.
  • Keep your home cool by opening windows to catch the breeze and keeping curtains or blinds closed to keep the sun out.
  • Do not to leave anyone, particularly children, older people or pets alone in parked cars.
  • If you must be out in the heat, limit your outdoor activity to morning and evening hours, and try to stay out of the sun.
  • Seek medical help if feeling weak or dizzy or if having intense thirst or a headache.

Contact Healthline for advice from trained registered nurses by calling freephone 0800 611 116.


Reducing the impacts of heat

The effects of heat may be exacerbated in cities, due to the urban heat island (UHI) effect, the livelihoods and wellbeing of non-urban communities can also be severely disrupted during and after periods of unusually hot weather.

The Ministry of Health (Manatū Hauora) recommends that Heat Health Plans be developed by emergency services, local government agencies and critical infrastructure providers on how their organisations will respond together and reduce the incidence of heat related illness. In 2018 Manatū Hauora released a Heat Health Plan Guidelines and Information to support health and community service providers, the health sector, local government and other community organisations prepare their own heat health response plans.

Reducing the impacts of extreme heat does not rest with emergency service providers alone. Measures to counter the effects of heat include:

  • planting trees as shade structures and for their cooling effect;
  • providing drinking-water fountains in public spaces;
  • including water-features in public spaces; and using natural materials for construction (such as wood).


Latest research

Massey University’s Environmental Health Indicators programme has published an updated factsheet on extreme temperature days (the hot & cold days indicators). Here are the key facts:

  • New Zealand experienced an average of 33.1 hot days, where maximum temperatures exceeded 25CM and 16.1 cold days, where minimum temperatures were below CBC in 2022.
  • In 2022, hot days were most common in Territorial Authorities (TAs) in the north and east of the North Island.
  • In 2022, cold days were most common in the southern and central South Island, and also the central North Island.
  • Higher concentrations of vulnerable populations live in areas prone to hot days, particularly Māori, young children and people living in higher levels of deprivation.

For more information read the latest factsheet.


Further information