
Updated EHINZ reports reveal that large numbers of waterborne disease cases are not receiving the necessary follow-up investigations.
EHINZ is a unit within Te Kunenga ki Pūrehuroa Massey University’s College of Health that provides information and intelligence on how the environment affects the health of Aotearoa New Zealand's population.
The EHINZ team has recently updated its surveillance reports on cases of waterborne diseases, including campylobacteriosis, giardiasis and cryptosporidiosis. These diseases, which are often transmitted through contaminated water, have been linked to both untreated drinking water and recreational water sources like lakes, rivers and the sea.
The updated reports reveal a worrying trend: large numbers of waterborne disease cases are not receiving the necessary follow-up investigations, particularly in relation to their potential connections to water contamination.
EHINZ Senior Intelligence Analyst Patrick Hipgrave says that while public health providers are required to report all relevant risk factors for each case of waterborne disease to the Institute for Environmental Science and Research (ESR), much of this crucial data goes unreported.
“Auckland's public health services, in particular, have shown consistent underreporting, with risk factor data included in only about 2 per cent of campylobacteriosis reports. This lack of thorough data collection means that many waterborne illnesses—potentially linked to untreated drinking water or recreational water—may go unrecognised and their causes unaddressed, within the healthcare system.”

EHINZ Senior Intelligence Analyst Patrick Hipgrave.
Untreated drinking water and waterborne diseases: a hidden threat
EHINZ's latest findings include 419 notifications of campylobacteriosis, 90 of cryptosporidiosis and 88 of giardiasis in 2023, where untreated drinking water was identified as a potential risk factor.
However, these numbers are likely to be underestimates, as risk factor data was missing for the majority of cases. In 2019-2023, no health district reported sufficient risk factor information for a reliable calculation of disease rates linked to untreated drinking water, which Mr Hipgrave says suggests the true scale of the issue is much higher.
“Despite the large number of reported cases, untreated drinking water as a risk factor was only noted in about 10 per cent or less of all notifications in 2023. In fact, more than two-thirds of notifications for these diseases had no risk factor information at all. EHINZ estimates that, based on the available data, there could be as many as 1,234 additional cases of campylobacteriosis, 120 cases of cryptosporidiosis and 171 cases of giardiasis in 2023 that may be linked to untreated drinking water.”
A call for better data and accountability
Mr Hipgrave says the lack of risk factor information hampers the ability to assess the true impact of untreated drinking water and recreational water on public health. These gaps in data are especially problematic for public health authorities trying to track the effectiveness of water safety measures and prevent further outbreaks.
“For instance, the 2016 contamination of Havelock North’s drinking water, which led to a major campylobacteriosis outbreak, highlights the potential severity of untreated water sources. The outbreak, which involved 964 confirmed cases, may have affected as many as 8,320 people in the town—demonstrating how such diseases can spread quickly when water sources are compromised,” he explains.
EHINZ also noted that completion rates for risk factor reporting have remained low across the country, with some districts showing risk factor completion rates as low as 1 per cent. In particular, Auckland’s public health districts (Waitematā, Auckland and Counties Manukau) continue to lag behind in data collection, leading to skewed disease rates in those areas.
Recreational water: a less recognised source of infection
In addition to untreated drinking water, EHINZ's report also examined recreational water as a potential source of disease.
In 2023, 103 notifications of campylobacteriosis, 26 of cryptosporidiosis and 35 of giardiasis were linked to recreational water contact. However, similar to the untreated drinking water data, these figures are likely to be underreported due to incomplete or absent risk factor information.
“While the COVID-19 pandemic may have contributed to lower reporting rates (due to reduced outdoor activities), the long-standing issues with risk factor reporting mean that even pre-pandemic data was incomplete. In total, recreational water-related disease notifications made up less than 5 per cent of all reported cases of these three diseases,” Mr Hipgrave explains.
EHINZ highlighted that, when considering the missing data, there could be an additional 215 cases of campylobacteriosis, 20 of cryptosporidiosis and 48 of giardiasis in 2023 potentially linked to recreational water exposure.
Completion rates for risk factor information vary significantly across New Zealand's health districts. Health districts such as West Coast, Lakes and Wairarapa have demonstrated relatively high notification rates with complete risk factor data, whereas the Auckland region has shown consistently poor data collection. The varying standards across different public health services (PHS) make it difficult to draw reliable conclusions from the data, highlighting the need for improved reporting practices nationwide.
The need for improved surveillance
As New Zealand continues to grapple with the challenges of monitoring waterborne diseases, Mr Hipgrave says stronger accountability and better data collection practices across all public health districts is needed.
“The ongoing gaps in risk factor reporting not only obscure the true extent of waterborne disease transmission but also limit the country's ability to address these public health threats effectively.”
EHINZ's surveillance relies on data from ESR’s EpiSurv system, which tracks notifications of gastrointestinal diseases reported by public health services. However, because many cases are missing crucial risk factor information, the true number of waterborne illnesses in New Zealand may be far higher than currently reported.
Read the complete reports on untreated water and recreational water.
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