Tummy upsets are a more serious problem than previously thought, causing 4.6 million bouts of illness a year, according to the first national survey of their prevalence.
At 1.1 bouts of acute diarrhoea and/or vomiting a year per person on average, New Zealand has a higher rate than Australia on 0.9, says the study by the Institute of Environmental Science & Research.
"The results of this study demonstrate that acute gastrointestinal illness poses a significant burden on the New Zealand community, which is markedly higher than previous estimates," says the study.
More than 3500 people were asked in the telephone survey if they had had a bout of diarrhoea and/or vomiting in the preceding four weeks, excluding non-infectious causes.
Nearly 9 per cent said yes, equating to 4.6 million cases a year nationally.
The study confirmed the significant impact on the economy, finding that the illnesses accounted for about 5.2 million days off work a year by sufferers and those caring for them.
Previous estimates have indicated the total cost of all food-borne illnesses is $55 million a year, including almost 500,000 sick days _ but the latest study covers all causes of acute stomach upsets, including waterborne disease and person-to-person infection in addition to food-poisoning.
In the phone survey, the majority of those who had fallen ill reported vomiting and/or having diarrhoea for two days. About half had recovered by day two, and 80 per cent by day four.
The number of cases notified to health authorities in the survey period equated to 0.5 per cent of all the cases in the survey _ slightly higher than Australian and Canadian notification estimates but lower than for England.
Donald Campbell, principal adviser on public health at the Food Safety Authority, which commissioned the study, said that prior to the survey, authorities had to rely on estimates.
"The problem is obvious: few people who get diarrhoea or vomiting go to the doctor and, of those that do, the cause of infection is not always identified because many gastrointestinal illnesses take days or even weeks to cause symptoms."
But further research was needed to determine what proportion of the overall problem was due to food-poisoning.
The Health Ministry's public health director, Mark Jacobs, said: "Acute gastrointestinal illness is a significant problem in New Zealand _ and the world _ and generally under-estimated in its importance."
Measures the Government was taking to reduce it included last year's drinking water legislation and a public health bill which included a strengthening of disease-reporting requirements on doctors.
* 4.6 million cases of acute gastrointestinal illness a year nationally.
* That's about 1.1 bouts of vomiting and/or diarrhoea for each person.
* But children under 5 have more cases than average and adults have fewer.
* Maori have more than non-Maori, but income does not affect prevalence.
* This illness causes an estimated 5.2 million days off paid work annually by sufferers and their carers.
* 90 per cent of the sick missed work; the median absence was two days.
* 35.5 per cent of the sick sought help from a health practitioner.