The Hawke's Bay District Health Board is warning Havelock North residents to continue to boil their water because it may contain a bug that isn't affected by chlorination.
Hastings District Council has chlorinated the water and residents are drinking bottled water after thousands got sick from campylobacter.
Today the DHB warned they were still testing for another parasite that causes the same symptoms.
"The cryptosporidium is not responsive to chlorination," acting Medical Officer of Health Dr William Ranger says.
He says there is ongoing testing of faeces specimens and water samples for cryptosporidium and giardia and they're still waiting on the results.
"The incubation period for those diseases is longer than campylobacter so we are carrying out enhanced surveillance."
Dr Ranger says some peoples' specimens have tested positive for cryptosporidium and giardia as well as campylobacter so further work is needed to establish which bug is causing the illness.
Hastings Mayor Lawrence Yule says they plan to have a UV filter in place for Havelock North's water supply by Friday which will eliminate all bugs.
This afternoon the Government will announce its terms of reference for an inquiry into the water contamination that affected more than 4000 Havelock North residents.