UPDATED 12.54pm The extent of Havelock North's gastro outbreak has been revealed.

The Hawke's Bay District Health Board has done a survey of residents and CEO Kevin Snee said it shows the bug has had a devastating impact on the community.

"What it looks like, from that survey, is that 50 per cent of households in Havelock have been affected. The numbers of people who've been affected are around 3200."

Mr Snee said they are starting to see reductions in ambulance call outs, and the number of people showing up to the emergency department.

"We have now 18 patients who are in hospital still. One patient is intensive care and one has come out of intensive care."

Hastings mayor Lawrence Yule is preparing to find out whether the council is legally liable for the country's worst water contamination.

The council is still trying to pinpoint the source of the camplyobacter in Havelock North's water.

SEE ALSO: Response slow to Havelock North gastro outbreak - District Councillor  

The Hastings District Council will today name who is going to lead a major investigation into what caused the contamination, and whether their immediate response to the crisis was adequate.

Tests due back by Friday will determine whether it's from avian, cattle or sheep faeces.

Lawrence Yule said if the council has been negligent in any way, it will come out of the thorough investigation due to take place.

"Clearly, if there's been negligence by the council somehow, there'll be questions asked about our liability," he said. "What we need to do is find out the cause and how this happened and who was responsible."

Te Mata Mushroom Farm in Havelock North is under the spotlight as a possible source, as it uses chicken manure to grow its produce.

Spokesman Michael Whittaker said nothing on their farm indicates they have issues.

"There's been no breach of our effluent pond. Chicken manure sits in a dry enclosed shed and there's been no breaches of that. There are no waterways close to that." 

The farm's had all their bores independently tested.

"There's zero e coli on the farm so as far as we're concerned, it's a mystery like everything else," Mr Whittaker said.

He said they have 124 full time staff and only one person is ill, so their own drinking water is fine too.

Horticulture New Zealand wants to reassure the public it's safe to eat fruit and vegetables grown in Havelock North.

Chief executive Mike Chapman said the contamination is definitely not in the region's food supply.

"For many, many years we've run very good food safety systems. These food safety systems run all the time and with those safety systems in place there is no need for concern."

He said many growers in the region are holding off on picking their crops as a precaution.

Council water engineer Brett Chapman said they will have a mobile UV treatment plant operating in Havelock North next week, which will clean the town's drinking water.

He said the plant basically nukes all the bugs in the water.

Mr Chapman said because they don't know the source of the contamination yet, the water will continue to be chlorinated and residents still need to boil it.

MORE: Govt showing no leadership over Havelock North contamination crisis says Labour

But some Havelock North residents don't know if they'll ever be able to trust the cleanliness of the town's water again.

A kidney transplant recipient, who lives close to the town, said she got violently ill a week ago - well before any public warning went out about the water on Friday.

The lady, who didn't want to be named, said many still feel vulnerable, more so if the source of the contamination isn't found soon.

"The irony of it all is that as I was drinking tablets for the antibiotics, I started to feel sicker drinking water, and I couldn't understand it. It felt like the water was making me sick."

Selwyn District Council had it's own smaller outbreak when 125 people fell ill in Darfield in 2012.

Council asset manager Murray Washington had some practical advice for Hastings Council in tackling the incident's aftermath.

"Keep communicating, keep those blinds open, undertake a through review of what has happened, implement any recommendations that come out of that review."