Hawke's Bay's water operators are checking the region's supplies for contamination of Cryptosporidium.
Health authorities have issued a warning following the diagnosis of 45 people over the past two months.
Medical Officer of Health Dr Nicholas Jones said the health protection team is analysing information to establish the cause of the outbreak - and expect there is more than one source.
"We've been in touch with the water operators and they don't think they've got problems at the moment, but obviously it's something we're going to be looking into in more detail," said Dr Jones.
