Test results for campylobacter, from a person who had a gastro illness and died on the weekend, were expected later this week.
Medical Officer of Health Nick Jones said campylobacter was usually not highly contagious and typically spread "by poo getting into your mouth". Washing hands before eating and after toileting was critical.
Many in the community have questioned why it took so long to be informed.
A council indicator test taken on Thursday returned positive on Friday morning and was sent back to be re-tested.
Council water supply manager Dylan Stuijt said positive indicator tests returned negative after re-testing 70 to 80 per cent of the time.
The DHB was aware of a small number of positive samples for campylobacter and sent an email before 1pm Friday requesting a meeting with the council at 2pm. By 3pm it was decided to chlorinate the water supply. The DHB sent out a media statement about 6.30pm alerting people of widespread vomiting and diarrhoea and advising them to boil water.
Jones said it was difficult to see how it could have been done quicker.
In a separate incident a private water tanker driver operator provided a positive indicator test to the DHB on Thursday from water taken from a Havelock North bore. He was told to dispose of the water and sanitise his tank, because the presumption was the bore water was clean.
On Saturday E.coli was confirmed present in the water and an emergency response was launched. Only then were resthomes contacted.
Jones said by that time chlorination would have killed any campylobacter in the water.
Yule said it would be confirmed today who would lead the investigation. The Ministry of Health may also conduct their own independent investigation.
One ecologist says future outbreaks of waterborne diseases could be prevented if New Zealand farmers moved livestock away from waterways.
Massey University's Dr Mike Joy said New York City had chosen to do so in the 1990s and the result had been a "win-win for everyone".
Illness hits hundreds
• More than 1000 people affected.
• Two people in a critical condition in intensive care.
• Eight Havelock North schools closed.
• Some schools had 50 per cent of their roll sick.
• About 5800 homes in the affected catchment.
• One death at a rest home that could be linked to the outbreak.