Heatwaves can kill, there will be more of them, and district health boards, councils and businesses need to develop health plans to manage their effects, says the Ministry of Health.
Many countries already had heat health plans - including generally cooler nations such as Britain - but New Zealand did not, the ministry said.
New Zealand did not even have a formal definition of "heatwave" - although a guideline report issued by the ministry at the weekend used a World Meteorological Organisation definition, of marked unusually hot weather persisting for at least two consecutive days during the hot part of the year.
Australia is now experiencing a record-breaking heatwave.
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Noona, in western New South Wales, made history on Friday with a highest minimum temperature of 35.9C, and maximum temperatures have neared 50C in some places.
The weather patterns driving that heatwave have affected New Zealand's weather, bringing strong winds and temperatures in the high 20s.
The recently issued Niwa annual climate review said last January was the hottest month recorded in New Zealand, and 2018 was New Zealand's equal-second warmest year on record.
"Because effects of heat are associated with relative rather than absolute temperatures, even in New Zealand's temperate climate people can experience negative health effects with modest increases in seasonal temperature," the ministry said.
The report recommended local government and district health boards develop heat health plans and integrate those with existing emergency management plans.
As well as enabling health measures, proper monitoring of heatwave conditions could benefit transport, electricity and water providers and environmental planners.