A health and safety review is under way after three Waikato prisoners were infected with campylobacter.
A total of 62 prisoners at Spring Hill Corrections Facility told staff they felt unwell amid a mass illness that was believed to have started on Christmas Eve.
Acting deputy prison director Megan Tuhoro said three prisoners were confirmed as having campylobacter and had since recovered.
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As of Wednesday, no prisoners at the site have reported symptoms of gastroenteritis.
Tuhoro said as a precaution four prisoners with pre-existing health conditions were taken to hospital for assessment.
However, none were admitted and all four were cared for at the prison.
"Prisoners with campylobacter have been closely monitored by registered nurses and provided with, hydration, electrolyte replacement, rest and paracetamol for pain relief," Tuhoro said.
"The same care was also provided to prisoners who reported feeling unwell."
The source of the infection remains unknown and a health and safety review is under way.
"The site will continue to work with Public Health to determine the source of the infection," Tuhoro said.
"All prison meals are prepared in industrial kitchens under the direct supervision of Instructors working within the National Food Control Plan.
"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."
In addition to the review, the site took steps including:
• Ensuring that all food preparation policies and established kitchen practices are being adhered to.
• Providing information to prisoners regarding good personal hygiene and food handling practices.
• Reinforcing the importance to staff of thorough searches to detect food contraband, including homebrew.