Nearly 15 patients and six staff members of Timaru Hospital are extremely unwell due to vomiting and diarrhea.
Experts suspect it to be an attack of norovirus.
"Norovirus is always circulating in the community and there is always a risk that someone carrying the virus will visit a relative in a hospital or rest home and trigger an outbreak."
As a precautionary measure, the hospital is closed to visitors.
The hospital is still awaiting test results to ascertain the cause of vomiting and diarrhea.