Napier City's water supply will be chlorinated this morning after testing positive for E coli late yesterday.

Despite the water being chlorinated, Napier City Council's manager asset strategy, Chris Dolley, stressed that no boiling of water was necessary.

"We understand that residents may feel nervous, especially in light of previous water issues around Hawke's Bay, however we are confident that we are dealing with this positive test in the best way possible."

After the council's positive result back in February, the water team switched from standard testing to more accurate enumerated testing, which gave a reading of between 0 and 10.

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The level detected yesterday in Napier's water was measured as a 1, which was the lowest level determinable.

However, having experienced several transgressions since February the council, with the full support of the Hawke's Bay District Health Board, decided to urgently begin a programme of chlorination as a precaution.

Napier mayor Bill Dalton said the council took this matter extremely seriously and felt chlorinating the network for several weeks was the responsible thing to do.

"In the past, we've successfully isolated and chlorinated particular reservoirs. This time we will chlorinate widely, and for up to a month, in order to eliminate whatever bugs are affecting our water," Mr Dalton said.

All of Napier, with the exception of Otatara and Puketapu, will be chlorinated.

That means that the smell or taste of chlorine may be detectable in the water in Taradale, Bayview and the central suburbs of Napier from this afternoon onwards.

Mr Dolley said most of the country drank chlorinated water as a matter of course and because their supplies were treated they did not get positive readings for bugs such as E coli.

"Here in Napier it's a different story and with the benefits of untreated water comes a degree of risk.

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"Having said that, having such a high number of transgressions has come as a surprise to us all and along with this period of chlorination, we're rolling out a series of planned improvements to the entire water network, starting on Wednesday [today]."

He said these improvements would concentrate on modifying or completely overhauling the testing points - the places where water was drawn for regular testing - because these sites could be sources of contamination, which result in positive tests.

In the meantime, a secure chlorine dosing site is to be established in Taradale, injecting chlorine into the reticulated system 24 hours a day, for as long as is needed.

Mr Dolley said it was a huge effort requiring a great deal of resource, but the Hastings District Council was assisting with equipment, and health and safety and logistics were all on track for an early start today.

Daily testing will continue and schools, businesses and tourism operators are being notified of the chlorination.

A chlorine-free tap, drawing water from the unaffected Otatara and Puketapu area, will be established as soon as possible.

Napier residents can expect regular water updates via Facebook, radio and newspapers, and other channels as the chlorination programme continues.