Chlorine looks set to remain in Napier's water supply for "a few more weeks", bringing the total period of expected chlorination to over a month.

Chlorine first entered the supply in late May, after a positive reading for E.coli was returned at one of the Napier City Council's Park Island sites. This was a reading of 1 - the lowest level possible.

Despite there being no further positive E.coli tests, the council has said the continued chlorination is being used to disinfect the pipes.

Yesterday the city entered the fourth - and thought to be final - week of its water supply being chlorinated.

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While a fortnight into the chlorination it was expected the treatment would last a total four weeks, yesterday the council said it was not "in a position to remove chlorination".

"Safety of our town water supply remains our top priority and we are taking guidance from the drinking water assessor on when we can stop this programme of work," a council spokeswoman said yesterday.

She said it was likely the chlorination would continue for a few more weeks, with the decision to stop chlorinating made in consultation with the drinking water assessor, and the Hawke's Bay District Health Board.

Although it had previously been indicated chlorination would continue for only a month, chlorine levels were taking longer to stabilise than initially thought.

"We have never chlorinated the network before so it is a learning curve understanding the behaviour of our network," she said. "We have installed an additional chlorinator at the Bayview Reservoir to assist in chlorination all areas of the network.

Yesterday Napier mayor Bill Dalton said the chlorination was allowing the council to disinfect its water lines, "and then we'll be able to go back the clearest, cleanest, pure water supply in New Zealand".

"We'll only chlorinate it for as long as we have to, to make sure that we have a very clean and disinfected distribution network," Mr Dalton said.

Of the delay, council chief executive Wayne Jack said officials were waiting for the chlorination to stabilise in the network, adding "all that means is that the chlorination is doing its job".

He thanked residents for their patience, adding, "We're absolutely committed to providing the safest drinking water and, in the long term, in a non-chlorine state."

Last week the network's chlorination was discussed during a publicly excluded council audit and risk committee meeting - including the role drinking water assessors played in determining when the chlorination of the network could end.

This is the second time this year Napier's water has been chlorinated after positive E.coli readings.

Given the high number of transgressions recently, the council has also begun rolling out a series of planned improvements to the entire water network - including the inspections of 11 reservoirs, and a "full bore-head water-quality review" of eight bores.

This was said to be a "comprehensive" programme of work which would look at operations, maintenance and capital improvements, and would take several years to deliver in full.