It it was "highly likely" that the bug in the water that caused widespread illness in Havelock North was campylobacter, the Hastings District Council says.

All water tests have been clear since the chlorination of the drinking water supply started on Friday. Those tests have been carried out daily.

Chlorine is very effective at killing campylobacter and was put into the water supply at 5.30pm on Friday night.

Hastings mayor Lawrence Yule said staff worked through the night, drawing water through the pipe system so that chlorinated water would be right through the residential area by the following morning.

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On Saturday morning, rural residential properties with tanks filled from the town supply were chlorinated and schools' pipes were flushed to ensure all water systems, such as drinking fountains, had treated water through them.

Boil water notices remain in force.

Mr Yule said water tests carried out in Hastings central and Flaxmere yesterday came back clear.

"To provide reassurance, daily tests will be carried out on all supplies, both the bores and the reticulation systems."

"The water supplies are completely separate, but we wanted to put people's minds at rest. We can report that absolutely all tests from 19 points the bores and across the systems in Hastings and Flaxmere are clear," Mr Yule said.

An ultra violet light filter that will treat the Havelock North water is being loaned to the council while decisions on future treatment of the supply are made.

The offer of a portable plant has been made by Watercare, which manages Auckland's water supply.

Council water services manager Brett Chapman said ultra violet treatment is proven to remove the contaminants that can make people ill, and provides an additional level of treatment on top of chlorination.

Permanent treatment of the water would be required after this incident, Mr Chapman said.

Council would consider all options.

"But in the meantime it is really appreciated that Auckland can lend us this filter".

Free drinking water is available from tankers stationed at five points across Havelock North: at Te Mata Primary School, Lucknow Primary School, Havelock North High School, Havelock North Primary School and the New World Havelock North car park.