Testing of the water from bore 1 detected E.coli 39 hours after the pumps started and this continued to be detected for the next seven days.
Campylobacter was detected at bore 1 two days after the dye was introduced, and also found in the pond. This timing matched the time scale of the outbreak last year, the report on the tests showed.
The caucus also considered the possibility of the water travelling from neighbouring paddocks to the dry well and then to bore 1, and decided there was only a 20 per cent probability of this being the pathway.
Hastings mayor Lawrence Yule said that without pre-empting the ongoing inquiry he was happy that there was now a highly probable answer as to how the outbreak happened.
"It's helpful to have this answer and I hope it will restore public confidence that there has been no neglect."
He said that in the 35 years that bore 1 had been operating the only other instance of water contamination was in 1998.
"Now that we have found the cause we can move on to addressing issues like the chlorine in the water and getting bore 3 running with its UV treatment to ease the water restrictions.
"We have the UV system for bore 3 which can be set up for bore 2 if we need to, but at the moment bore 3 will be enough to provide enough water in the immediate future."
Meanwhile a Hawke's Bay Regional Council scientist has assured the Havelock North Water Inquiry panel that his actions were not part of a broader attempt by the Hawke's Bay Regional Council (HBRC) to blame the Hastings District Council (HDC) for last August's campylobacter outbreak.
At the fourth day of the inquiry hearings, barrister Matthew Casey, representing the HDC, said he would be putting a submission to the inquiry that when evidence was put to HBRC manager environmental scientist Stephen Swabey that went against what he thought, Dr Swabey's response was to refute that evidence in order to support his own theories.
Inquiry chair Justice Lyn Stevens picked up on this, and asked Dr Swabey directly whether his evidence was a concerted effort to support the theory that contaminated water went down the borehead.
"Is that something you alone have been advocating, or is it part of a broader attempt within the Hawke's Bay Regional Council to put the blame on the Hastings District Council?" he asked.
Dr Swabey responded that he had been pushing neither his own agenda nor the regional council's.
"We were very clear at the beginning to be open to all matters that might have caused the contamination at Havelock North."
Justice Stevens said this clarification was important given the timing of the HBRC's prosecution of the HDC, and the fact that the regional council did not have conclusive dye testing results at that time.
"I can't speak for the group manager or chief executive. I acknowledge this looks like it's deflecting blame from the regional council but this was not the intent, nor was it to pre-empt the inquiry."
"Senior management have allowed me to work independently, I have also consulted widely with outside experts," Dr Swabey said.