Claims that Hastings District Council knew water was contaminated days before a gastro outbreak in Havelock North are false and unhelpful, the mayor says.

One in two households in Havelock North have been struck down with the bug after testing on Friday picked up campylobacter in the water supply.

In total 3200 people have been affected, including nearly two dozen who ended up in hospital, two critically ill.

LISTEN ABOVE: Water treatment expert Ian Rabbitt speaks with Larry Williams

Hastings mayor Lawrence Yule said water tests on Friday morning picked up something suspicious, the water was chlorinated by 5.30pm and the bug dead through the whole system by 8pm.

"It's completely unhelpful for people to be saying we knew about this earlier," he said.

"Any speculation we knew earlier, or that we knew Tuesday or Wednesday, is completely false."

The latest figures, following phone surveys and door knocking, show around 3200 people have fallen ill since Friday.

"This has never happened in New Zealand before so it's a major national incident," Mr Yule said.

SEE ALSO: Half of Havelock North households affected by gastro outbreak

• Half of Havelock North households affected by gastro outbreak

Daily water testing is still underway and some bacteria is still being recorded in bore two, but tests of water once it has been chlorinated are clear, according to council staff.

Hawke's Bay District Health Board medical officer Nick Jones said action had been taken to fix the water quicker than national drinking water standards required.

"Because we had a hunch there was something going on ... we thought we were sufficiently concerned there may be something going on there," he told reporters on Wednesday.

"I'm absolutely convinced that saved a lot of people's health."

Officials believe the health system is coping well with the unprecedented outbreak but are continuing to urge people to be vigilant about washing their hands.

Mr Jones said people should wash their hands before and after preparing food, after going to the toilet or changing a baby's nappy, after caring for people with campylobacter and after contact with animals.

While campylobacter can't be spread through the air, it can be spread by swallowing the bacteria, through either contaminated water and food, or contact with infected people.

The notice to boil water will also remain in place until authorities are confident there is no other bug resistant to chlorination in the water.

HAVELOCK NORTH SCHOOLS TO REMAIN CLOSED UNTIL MONDAY

Meanwhile, primary and intermediate schools in Havelock North will stay shut until Monday.

Affected schools in the town have just released a statement.

Schools closed until Monday include Havelock North Primary, Te Mata Primary, Lucknow Primary, Havelock North Intermediate and Hereworth School.

This decision has been based on advice from Public Health officials and the ability of young children to stick to strict hygiene standards.

However, secondary schools including Havelock North High School, Iona College and Woodford House will reopen tomorrow.