The public is being urged to stay away from Waikato Hospital, unless it is unavoidable because of an outbreak of the highly infectious norovirus.
The hospital says 12 of the 16 patients in ward five have symptoms of norovirus, as does one staff member.
Ward five is a general ward.
The hospital says high occupancy rates and a ward closure are making it difficult to isolate and manage the affected patients.
It says there is a high number of people with gastro illnesses in the community, rest homes and within the hospital.
GPs and rest homes in the region are being advised not to send any patients with gastro illnesses to hospital if it can be avoided.
Norovirus is a highly infectious virus transmitted by hand to mouth, and symptoms include nausea, vomitting, diarrhoea and muscle aches.