Spring Hill prison is dealing with a mass-food poisoning that has so far affected 47 prisoners.

Acting prison director Megan Tuhoro said the first prisoner at the Waikato prison became sick on Christmas Eve. That was followed by 46 others who reported feeling sick.

One case of campylobacter has been confirmed. The working assumption was that the other 46 prisoners likely also had campylobacter.

Four prisoners with pre-existing conditions were taken to hospital for precautionary assessments but were not admitted and were back behind bars.

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"The source of the infection is currently unknown. The site will continue to work with Public Health to determine the source of the infection," Tuhoro said.

Istock Up close with campylobacter (File photo).

The Corrections Association said staff believe the confirmed case of food poisoning started with chicken salad.

Nurses were monitoring the situation and staff had been briefed on caring for sick prisoners, Tuhoro said.

"This includes close observation, hydration, electrolyte replacement, rest and paracetamol for pain relief."

All prison meals were prepared in industrial kitchens overseen by instructors.

MARK TAYLOR/STUFF Spring Hill Corrections Facility is dealing with a mass-illness.

According to the Corrections website the prison runs a hospitality training course in which up to 10 prisoners are embedded in the prison kitchen.

"There are clear and well-established quality and hygiene practices and policies to ensure that all food is prepared safely and to a high standard," Tuhoro said.

Potential infection sources for campylobacter included contaminated food or water, direct handling of animals, or cross contamination via substandard hygiene practices, she said.

Spring Hill Corrections Facility currently has 879 prisoners.

Corrections Association national vice-president Paul Dennehy said the cause of campylobacter could have been prisoners keeping food stored for later. But the food could have turned toxic bad in the heat.

According to Ministry for Primary Industries, campylobacter is New Zealand's most-common cause of food poisoning.

Symptoms include headaches, fever, diarrhoea, abdominal cramps and pain, and muscle pain.