The Ministry of Health has found Havelock North's water contamination cost about $21 million - with residents the worst affected.

Photo: RNZ / Jemma Brackebush

The campylobacter infection hit the town last August and afflicted more than 5000 people with illness, filling the hospital and potentially contributing to three deaths.

The investigators measured the next best thing that people could have been doing if they had not been sick. That and the value of direct costs added up to the total figure of $21,029,288.

Read the full report: (PDF; 1351kb)

A report commissioned by the ministry said some 5088 households were affected by the crisis, and the cost to each household was about $2440.

Those costs included the cost of people getting sick and being unable to go to work or school or carry out other tasks.

Some were unable to look after their children, while others had to drive all over town to visit doctors or to get fresh water or other supplies.

They also had to do far more laundry and cleaning.

This left the households to foot a bill of more than $12,420,000 making up the majority of all costs from the crisis.

Photo: Ministry of Health / Sapere research group

The report also said not all consequences of the outbreak could be quantified in monetary terms, with personal stress, loss of public faith in the water supply, and "scarring" of the community adding to the societal bill.

The report said about 25 percent of the population of Havelock North was aged over 65 based on the 2013 Census, and the town also had a large number of school aged children.

"The DHB .... is aware of a number of community-dwelling older people affected by campylobacter who have since become significantly more dependent on community support services, or who have required transfer to residential care," the report said.

"Both these scenarios represent potential rapid and significant progression of frailty."

Twelve people had to go to hospital with a variety of conditions which were hastened by campylobactyer infection.

They included hypotension, lower leg cellulitis, urine retention, delirium and pneumonia.

The report was at pains to note it was conducting an economic inquiry - using the principle of the "opportunity cost" - rather than a medical one.

It acknowledged there were many other costs that could not be measured.

The second biggest share of the costs, $4,133,080, was borne by local government. These included investigating the cause of the outbreak, providing boiled water and re-engineering existing water supply systems.

It also covered the cost of paying for an official inquiry, which made up about half the price.