A surge in stomach bug cases this summer is being put down to factors including the country's unprecedented heatwave and lax food preparation practices by holidaymakers.
In Auckland the number of notifications – some caused by food poisoning – increased by almost 50 per cent in January compared with the same month last year, while Northland faced a jump of more than 70 per cent.
The notifications were for nine different diseases which could cause gastro problems, including campylobacter, salmonella and giardia, as well as gastroenteritis itself.
Auckland Regional Public Health Service (ARPHS) medical officer of health David Sinclair said that while a rise in cases of stomach bug was expected in the region in January, notifications had been higher than usual.
"This is likely due to higher than normal temperatures, more people outdoors in the hotter weather and different cooking methods for example cooking outdoors while camping.
"Hot summer temperatures, especially during a heatwave are a breeding ground for bacteria like campylobacter and salmonella. These bacteria contaminate food and multiply in warm, moist conditions, which is exactly why meat is the ideal carrier for bacteria," said Sinclair.
However gastro was not always caused by food. Sinclair said it could also be spread through contact with animals, infected people and untreated water supplies.
January was the country's hottest month in 150 years, with average temperatures reaching a scorching 20.2C - 3C above normal, according to one of New Zealand's top climate scientists Dr Jim Salinger.
According to provisional data from the Institute of Environmental Science and Research's (ESR) Monthly Notifiable Disease Surveillance Report, Auckland's three district health boards received 433 notifications for stomach bugs including campylobacter, salmonella and giardia in January.
This compared to 290 during January 2017.
Northland, Bay of Plenty, Hawke's Bay, Taranaki, Whanganui, Wairarapa, West Coast, South Canterbury, Southern and Wellington's two DHBs also saw notifications increase.
Northland DHB notifications surged by more than 74 per cent, from 35 cases in January 2017 to 61 last month.
Hawke's Bay DHB had 47 notifications in January, compared to 30 in the same month last year - a more than 56 per cent increase.
Meanwhile, Wellington's two DHBs had a total of 151 notifications in January, nearly 40 per cent up on the 108 cases reported in the same month last year.
ESR public health physician Jill Sherwood said notification rates in Otago, Wellington, Hawke's Bay and Nelson Marlborough could also be affected by changes in laboratory testing methods in those areas, between January 2017 and 2018.
The changes meant more cases of infection were likely to be detected and in a more timely manner.
Auckland-based nutritionist Amy Judd said Kiwis could minimise the risk of getting food-borne illnesses by taking extra care when preparing meals.
"Particularly in summer – a lot of people tend to be a bit more social and have people around. I think one of the things that can go astray is simple things like hand hygiene," she said.
"The main mistakes people make is inadequate cooking or reheating or cross contamination, so [touching] food like raw chicken that can contain bacteria and [then] touching other things."
Judd said people including the elderly and pregnant women were more susceptible to food poisoning so needed to take extra care.
Nutritionist Amy Judd's food preparation tips
Follow the 4 Cs - Clean, Cook, Cover, and Chill.
• Wash your hands constantly while handling food, particularly raw chicken.
• Have separate areas and chopping boards for raw and cooked meats. This included making sure raw and cooked meats were not touching on a BBQ.
• Make sure food gets hot enough when you cook it. Food should be heated to 75C to kill any bacteria that could cause food poisoning. Reheated food should also be piping hot.
• If food has been out for less than two hours it can be put in the fridge. Food left out for more than two hours needs to be eaten immediately, while that left for more than four hours must be thrown away.
• Anything that is supposed to be cold should be kept at fridge temperature (normally 4C) or colder.
• Store raw meat at the bottom of the fridge to ensure it doesn't drip on other food.