It could be months before Havelock North residents get answers on how their water became contaminated, but criminal charges could be a possibility.

The government has launched an investigation into the contamination which has infected over 4000 people in the Hawke's Bay area over the last fortnight. The inquiry will seek to find out how the water was infected, how the crisis was handled, and the response to it.

Prime Minister John Key wouldn't say whether there will be charges until the cause of contamination is known, but any legal action would be decided by an independent body like the police.

"The inquiry doesn't of itself decide whether civil or criminal proceedings should be taking place because if they did, we would be in a police state," Key said. "It would be the government saying that happens."

Labour Party leader Andrew Little suggested that to make the wait easier for locals the investigators should make as much information public as they can.

"You would hope that at least some of the process would be held in public," Little said.

"I don't see any reason why people who have been affected by the outbreak can't say in a public hearing how they have been affected."

But Little said it was far too early for the Prime Minister to say anything about criminal charges.

"Having now raised the threat of criminal and civil proceedings, my fear is that you're now going to get people who think their going to be in the gun here, and will shut-up as much as they can."

Some scientists and the Green Party have called for the inquiry to assess the effect of farming on water quality, saying E. coli and campylobacter bacteria have been reported in other water supplies including Patea, Hanmer Springs and Christchurch.

Greenpeace is arguing the government needs to stop funding big irrigation schemes if it wants to prevent similar water crisis happening elsewhere.

Genevieve Toop, a Greenpeace agriculture campaigner, believes the crisis was likely caused by industrial farming which has been left to expand unchecked across the nation "which has to stop."

"The best thing the government could do is pull financial support from think big irrigation schemes, because these irrigation schemes will mean more industrial dairy farming, that means more pollution in our rivers, and more risk to our clean drinking water."

Green Party Co-Leader Metiria Turei's critical the inquiry won't cover land use.

"When it is well known that intensive agriculture is linked to declining water quality, and therefore could be a factor in this outbreak."

But the Prime Minister said the inquiry's terms of reference are very broad.

"And I don't think, as much as it suits the members own political agenda and her followers, that she should jump to conclusions about what the cause of the water concerns are."

Mr Key said he knows there's been speculation about dairy farming being responsible for the contamination, but he has received different advice.

An order to boil all water before using it is still in place, and Hastings District Council water engineer Brett Chapman said it is essential people do so, until told otherwise.

"We're meeting with a number of experts, health officials to determine the criteria that will need to be ticked off so that we can all sign up to the lifting that boil water notice," Chapman said.

After primary and intermediate age students were back in their classrooms yesterday, they were warned not to bring any water home from school as a precaution to limit the spread of illness.

Numbers revealed yesterday show Havelock North Primary School had 46 students away with gastro, Te Mata Primary School had 35 sick, Lucknow Primary 22, and Hereworth School nine.

Hereworth School principal Steve Fiet explained that more than 3000 bottles of water had been donated to the school, which was "a phenomenal amount."

"We anticipate that will last us somewhere at least a couple of weeks."

Fiet believes it's lucky that more children weren't struck ill, but is thankful there aren't more empty desks.

"I was prepared to have a larger number of boys away ill so it was a welcome relief to see so many faces fit and well," he said.