Health authorities are warning people to take extra care in the heat over the next week, following the death of a Christchurch woman with multiple sclerosis on Wednesday.
Chief Coroner Judge Deborah Marshall ruled that the woman had died of hyperthermia, on a day where Christchurch hit 32C.
"I feel it is important to remind people of the dangers of overheating due to the high temperatures expected in the coming days and to take all necessary precautions," Marshall said.
Neil Woodhams, vice president of the Multiple Sclerosis Society of New Zealand, said anyone with MS is generally well aware of the dangers of heat.
"People with MS suffer from high heat to a greater extent than the normal population," he said. "It tends to make their limbs less able to work, and it increases fatigue."
Those with MS are also less able to regulate their own body temperature.
The society has tips to deal with the heat on its website.
"It's all of the normal things people ought to do to keep cool, but it's just more important that people with multiple sclerosis do it - having cold drinks, ice packs, staying in the shade, using air conditioning if you've got it," he said.
"It's particularly important that people with multiple sclerosis take the usual steps to keep as cool as possible over the next few days because there's a pattern of very high temperatures coming up."
Research from Australia - where temperatures regularly rocket past 40C - has found increasing body temperature by less than half a degree can cause worsening of neurological symptoms in people with MS.
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Canterbury Medical Officer of Health Dr Alistair Humphrey said older people, children and those with underlying medical conditions are most at risk from the impacts of heat stress.
Extreme heat could affect blood pressure and hydration, Humphrey said. He urged people to seek help if they felt dizzy, weak or had intense thirst or a headache.
The MetService has predicted Queenstown, Wanaka and Alexandra could reach "heatwave status" by Sunday, meaning the maximum temperature will have been 5C hotter than normal for five days in a row.
Niwa warned temperatures could possibly approach 40C next week.
The North Canterbury town of Waiau recorded a high of 37 degrees on Thursday - the warmest temperature recorded in New Zealand in almost seven years.