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.