The disease which struck down hundreds of people in Havelock North was most likely caused by campylobacter, the Hastings District Council says.
The bacteria is found in the gut of animals and birds and transmitted to people through contaminated water or food. The council has yet to find out how it got into the town's water supply last week, causing what's being labelled the largest such incident in modern New Zealand.
More than 1000 people have been struck down by the gastric illness, with schools shut and businesses forced to close.
The council says all water tests have been clear since the chlorination of the drinking water supply started on Friday. Those tests have been carried out daily. However boil water notices remain in force.
Hastings mayor Lawrence Yule said tests carried out in Hastings central and Flaxmere yesterday came back clear.
"Daily tests will be carried out on all supplies, both the bores and the reticulation systems. The water supplies are ... separate, but we [want] to put people's minds at rest."
It is the third time in three years there has been bacterial contamination in the same supply, which comes from bores drilled into the Te Mata aquifer 20m below ground.
E. coli - a bacteria found in faeces - was discovered in the groundwater supply in both 2013 and 2015. After last year's contamination, the water was chlorinated and the bore responsible - known as "bore 3" - was closed, and an investigation began.
The investigation is still not complete, with council documents showing its Works and Services Committee was yet to be informed of the possible causes of the previous contamination.
The investigation was being undertaken by environmental consultants Tonkin & Taylor. Bore 3 would remain closed until the report was complete, the documents said.
Bore 3 is less than 200 metres from bores 1 and 2 which are at the centre of the current incident. All are on Brookvale Rd, northwest of Havelock North.
Yesterday council officials said they could not yet tell the cause of the latest contamination, although it was likely caused by surface water and linked to a recent rainfall event.
The council said it would take experts some time to work out the cause of the latest incident, but agricultural and cropping activities were potential sources.
In Parliament, Labour deputy leader Annette King criticised the council's and the Government's response to the outbreak. She said despite people reportedly feeling ill last Tuesday, action was not taken for three more days. It was only yesterday that an outreach programme was implemented, she said.
"Why have we waited so long?" she said.
Acting Health Minister Sam Lotu-Iiga said it was important to show concern for the people affected, rather than playing politics.
"Now is not the time for [pointing] the finger. Now is the time to say, 'what can we do to assist', and that is what is being done."
- Additional reporting Hawke's Bay Today