The New Zealand government has launched an inquiry into the contamination of a regional water supply that has left thousands of people sick with vomiting and diarrhoea.

The outbreak of campylobacter bacteria, a form of gastroenteritis, has affected around 3,000 people, officials said.

About half of those are in the town of Havelock North on New Zealand's North Island, about 250 km northeast of the capital Wellington, where schools have been closed and medical facilities overloaded.

"The situation in Havelock North is concerning," Minister for Health Jonathan Coleman said in a statement. "It is important that the public has confidence in their water supply."

Such outbreaks are rare in New Zealand, where tap water is usually safe to drink. Officials said chlorine was being hastily added to the water supply for North Havelock and the surrounding region.

New Zealand touts its clean, green image abroad under a "100 percent pure" campaign to attract tourists, as well as to highlight its food and water safety as a hallmark of its premium export products.

The government said the investigation into the incident would focus on the causes of outbreak and the response.

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