People living in hot areas of the country should avoid drinking alcohol while temperatures skyrocket, according to the National Emergency Management Agency.
Parts of the South Island are basking in the heat, with temperatures in the high 20s expected across most regions.
Christchurch and Timaru, as of early Friday afternoon, had tipped the 30C mark.
"Stay safe in the heat," Civil Defence said in a statement.
"Drink plenty of water and avoid alcohol and caffeine."
Heavy drinking during hot temperatures can cause dehydration and heat stroke, experts say.
Caffeine, meanwhile, should only be consumed in moderation on hot days to avoid dehydration, according to scientists.
Te Whatu Ora/Health New Zealand's Cheryl Brunton urged people to take sensible precautions in the heat.
"Over the next few days, if your plans involve being outdoors, it's important that you stay out of the sun where possible, ensure people and pets aren't left alone in stationary cars and avoid extreme physical exertion - especially during the hottest part of the day," said Dr Brunton, Canterbury's medical officer of health.
"While the elderly, infants and children, women who are pregnant, people suffering from chronic, acute and severe illness are more at risk, heat can affect us all and overheating is a condition that can prove fatal," Dr Brunton added.