The regional council was found to have failed in not meeting its responsibilities under the Resource Management Act to protect the Te Mata aquifer, which was the first and most critical step in a multi-barrier approach to ensuring safe drinking water.
The council's knowledge and awareness of the aquifer and the risks fell below the required standards, including the state of numerous uncapped or disused bores in the area.
Mr Stevens said the regional council also failed to monitor compliance with the conditions of permits granted to Hastings District Council to use the bores.
When asked if he agreed with the inquiry's findings that the council failed to care for the public's health, Mr Graham said he would like to read the inquiry's report further, but at this stage would "emphatically say that we have performed our duty to protect the public health".
The inquiry found that it was highly likely the outbreak was caused by contaminated run-off from a sheep paddock after heavy rain on August 5 and 6 entered the Mangateretere pond near Brookvale Bore 1 and 2.
Water from the pond entered the aquifer and flowed across to Bore 1 where it was pumped into the reticulation.
When asked if he accepted this as the cause, Mr Graham said he would need to read the inquiry's findings before commenting.
He did add the council "spent millions protecting the aquifer" and that he was "totally confident in the Heretaunga aquifer being safe for drinking".
The "dysfunctional" relationship, and a critical lack of collaboration between the two councils was highlighted as another failing.
While their relationship prior to the outbreak might not have directly contributed to the outbreak, "at the very least it resulted in a number of missed opportunities".
"The uptake of such opportunities might well have prevented the outbreak," he said.
Mr Graham said the relationship between the two councils was complicated due to his council's statutory responsibility.
"From time to time we run into conflict with councils... and in this instance the Hastings District Council around this water issue. Whether we handled that as well as we might politically or social, I think probably we didn't."
While his friendly relationship with Hastings mayor Lawrence Yule meant the two could "iron some of these issues out", he said they had failed to do so on this occasion.