The Whanganui District Council has given an assurance that providing safe drinking water is given its highest priority.
The reassurance comes in the wake of the outbreak of gastro illness in Havelock North area after the water supply there was found to be contaminated.
Two elderly people are in Hawke's Bay Hospital's intensive care unit in a critical condition, both with a gastro illness. A death at a Havelock North rest home may also be linked to the illness.
Meanwhile a number of schools have been closed and hundreds of school students were absent because of the disease on Friday.
The outbreak has been linked to an underground bore which tested positive for E. coli.
Mark Hughes, Whanganui District Council infrastructure manager, said the local supply was monitored on line "24 hours a day, seven days a week".
Mr Hughes said multiple samples were constantly taken from across the district and fully analysed at an accredited laboratory.
"Our water meets the highest quality of the New Zealand drinking water standards with an Aa rating," he said.
Water for the urban area comes from artesian bores at Kai Iwi, Westmere and Aramoho and is constantly treated. It's only after treatment that it is made available to consumers. And he said all rural supplies are also chlorinated and rigorously sampled and tested.
"Hawkes Bay and Napier also have artesian bores but they don't chlorinate their water. They're the only supplies in New Zealand that aren't chlorinated," Mr Hughes said.
Yesterday's edition of the Hawke's Bay Today newspaper carried a full-page apology from the Hastings District Council, signed by Mayor Lawrence Yule and chief executive Ross McLeod.
The statement outlined how the Havelock North water supply was tested on Tuesday, August 9, and found to be clear. Then a further test on Thursday, August 11, found a presence of E. coli. This result was available on Friday - about the same time discussions began with the district health board over "patterns of illness" starting to emerge.
The statement said chlorination of the water supply would continue indefinitely and a "major investigation" was under way into identifying the cause and ensuring it did not occur again.