Auckland health authorities are looking into whether sewage is to blame for spikes in people contracting stomach bugs in Auckland after every storm.
The Auckland Regional Public Health Service told the Herald it had begun a study on why it is seeing an increase in diseases like cryptosporidium and the potentially serious E coli (VTEC).
The public health service said it was working with Auckland University, Niwa, Auckland Council and Watercare on the study.
In an average week, the region usually saw between 60 and 75 cases - but that often rises to more than 80 after a storm - based on results of specimens analysed by labs.
David Sinclair, Medical Officer of Health and public health medical specialist, said he believed the number could be higher because most people with gastroenteritis symptoms did not see a GP.
"(We are) looking at researching the cause and ways of analysing the information to identify possible causes of a moderate increase in some types of gastroenteritis," Dr Sinclair said.
"This scientific investigation is at the exploratory stages and the first step is to look at the number of infections associated with rain versus comparable seasons without rain and to research potential risk factors."
The investigation will cover water contamination in streams, low-lying and coastal areas from run-off and whether there is sewage contamination of stormwater.
Wastewater systems in older parts of Auckland were designed to overflow in heavy rain to stop sewage flowing back into homes, and after big storms often things like tampons and toilet paper get washed up.
Some, like Meola Creek in West Auckland, have an average of two overflows a week.
"The overflow situation has been of concern for us for a long time, particularly the effect that it has downstream and as the contaminated water flows to the coast," Dr Sinclair said.
The Auckland Council has advised people to avoid stream mouths and stormwater outfalls for 48 hours after heavy rain. Beaches and rivers most at risk are listed on the council's website.