Prime Minister John Key says charges could be laid after the gastro outbreak in Hawke's Bay.

More than 4000 people are thought to have been struck down by Havelock North drinking water contaminated with campylobacter.

Key is backing the Hastings District Council, saying it acted as quickly as it could. But he told Newstalk ZB's Mike Hosking there could be court action, as it's not clear whether the contamination contributed to the death of a woman in a rest home.

He says that court action could involve civil or criminal charges.

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The Ministry of Education says all Havelock North's schools and early childhood centres are expected to be open today.

After the rolls are taken at Havelock North Intermediate this morning the students will be gathered for an assembly.

"We'll just talk through hygiene basically. Over the next little while we'll have the Red Cross in during the day at points. We'll be talking about using the hand sanitisers and washing hands properly when using the loo," said Principal Julia Beaumont.

At the weekend, the Hawke's Bay DHB said interim results from the Institute of Environmental Science and Research suggests contamination from cattle and other four-legged farm animals may have been in the water.

Medical authorities noted that there was a marked decrease in the number of patients presenting with gastro bug symptoms.

Test results from the Hastings water supply came back clear yesterday afternoon after earlier results showed an anomaly, put down to sampling irregularities.

The Council said today's results mean the Hastings supply, which also provides water to Flaxmere and Bridge Pa, can continue to be considered safe to drink, however, it will continue to chlorinate the supply in the short term.

Dr Bridgette Sullivan-Taylor, an extreme event expert at Auckland University, criticised the official reaction to the outbreak, saying she was "staggered" at how quickly the disaster snowballed out of control.

"Better planning could have minimised unintended consequences and downstream effects that make the disaster worse. The fact that contaminated water was brought into Havelock North in a tanker is staggering."

However, the district's Mayor Lawrence Yule maintained he didn't know how much quicker they could have acted.

"We're talking hours here, I'm not sure how much faster we could have gone. The critical thing is that we now know that people are likely to have been getting ill on Monday, yet we had a clear test on Tuesday," Yule said.

"Clearly if we had known something was happening on Tuesday, with the water, we may have acted on Tuesday but we had no indication anything was wrong."

Napier MP Stuart Nash said it's hard for locals to trust the water infrastructure again when it's still so unclear what went wrong.

"We're not looking for people to blame, there's no doubt about that, but it's hard to know what to do or where to look next."

- With Newstalk ZB