This tank has been shut off, meaning water restrictions now apply to Napier residents.
Napier Mayor Bill Dalton classed the positive result - which showed an E.Coli reading of 1.1 - as "absolutely marginal".
"Anything under one is regarded as not an issue so its 1.1 in one very small tank, in the whole network," he said.
Hawke's Bay District Health Board chief medical officer Dr Nick Jones agreed the 1.1 result was very low.
A Hawke's Bay District Health Board spokeswoman said there had been no increased reports of sickness in the Napier area. The DHB confirmed no boil water notice was required as the e-coli reading was extremely low.
Napier City Council's Manager Asset Strategy Chris Dolley said the reservoir was made of two tanks, with the larger returning "consistently clear results".
The small tank would not be used for city water supply until further notice. Before it was shut off, Mr Dalton said the water from the tank fed into the main system.
"So it could have potentially covered a wide area, but the fact is it is such a low level of possible contamination that it really is not an issue."
An investigation into the source of the E.coli is underway.
Mr Dalton said there were no early indicators of what the source might be, adding the low levels of E.Coli meant it would be "very, very hard to determine the source".
He confirmed there were no indicators of any other harmful pathogens in the water.
Council chief executive Wayne Jack said the cause of the E.Coli was "a little bit unusual" as tests on the rest of the network had returned clear.
Although he said the result was "disappointing", he lauded the council's quick actions.
While national standards allow the council to wait for a second positive result before taking action, the council said given the "learnings" from previous water situations they acted conservatively and immediately.
"We were very quickly on the measures to contain it within one small tank within the network," he said.
"It is still counted as being positive but we've taken actions and the teams done a great job to contain it and get onto it so quickly."
Mr Dalton said their reaction showed "that we have system in place if there are any potential risks to the health of our ratepayers that we can act immediately."
National water standards dictate that chlorination should continue until three negative tests are returned.
This means residents would continue tasting chlorine in their water for the meantime, as Mr Jack said the earliest the three tests could be returned would be early next week.
When asked if council were concerned about the impact this event could have on this months annual Art Deco festival, Mr Jack said they hoped the chlorine would be out of the network by then.
"And the network will be back to the standard the people of Napier have come to enjoy."
The council were installing a chlorine-free tap at Taradale Park, which was expected to be completed by midday today.
With the Enfield Rd tank - which houses approximately a third of the Enfield Road reservoir's capacity - now out of commission water restrictions will apply, meaning restricted periods of use for hoses and sprinklers.
Mr Dalton said the restrictions would stay in place for "as long as they have to".
"The issue with water restrictions is we've been struggling for some time because ...apart from cricket days we've had no rain. We've been managing to get through but it's been only a credit to the management of our team."
"We are asking for a bit of support with this and hoping people will be understanding under these circumstances" he said.
Council staff are working through the weekend and updates will continue when available. Daily testing is continuing. A full update on the weekend's e-coli sample test results will be available on Tuesday February 7th.
-If anyone is feeling sick with diarrhoea and/or vomiting they should see or call their health professional or call Healthline 0800 611 116.