Dust may be responsible for the widespread chlorination of the Napier water supply.

After being chlorinated for the past couple of weeks, the final dose was delivered to the Enfield Rd reservoir on Tuesday.

The water supply was chlorinated immediately after a reading of E.coli was detected in one of the reservoir's tanks this month. Later results showed a reading of 1.1 - anything under 1 is classed as safe to drink.

Although the contamination source had not been found, a Napier City Council spokeswoman said staff had a "very high level of confidence" the E.coli reading came from dust in the water supply.

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"Particularly since the reading was extremely low, this is consistent with the presence of dust," she said.

It was believed recent strong winds had blown dust into the tank, and the council would be installing new air vents in the reservoir as soon as practicable.

MetService meteorologist Brian Mercer said there had not been any strong or gale winds in the week before February 2, but there were "fresh winds" - when the mean speed exceeds 31km/h.

These were recorded overnight on January 26, twice on January 27, and all day on January 31.

The council had not yet "isolated the costs" relating to the incident, because all the testing and cleaning came under the annual water maintenance budget. There was also a contingency for cleaning and chlorine as part of the budget.

Napier mayor Bill Dalton declined to comment.

The chlorination programme had ceased because clear readings had been taken from the tank since being inspected, cleaned, and vacuumed by a professional dive squad, council manager asset strategy Chris Dolley said.

"It may take a further 24 to 48 hours for all the chlorinated water to make its way out of the system, so some people might still taste or smell the chlorine until late Thursday," he said.

Yesterday a Ministry of Health spokeswoman said for chlorination to cease the water supplier - the council - would be expected to investigate the reason for the problem and correct any faults found during the investigation.

The decision to stop chlorination sits with the water supplier, but is done in consultation with the Public Health Unit -drinking water assessor - at the Hawke's Bay District Health Board.

- For those unable to tolerate the chlorinated water supply, four chlorine-free taps remain in place at Anderson Park, set up on York Ave next to the light-blue Girl Guide Hall in Anderson Park.

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