Six patients at Nelson Hospital are in isolation with gastroenteritis and measures are being put in place in bid to stop its spread.
Tests are being conducted by the ESR to determine the exact cause, Nelson Marlborough District Health Board says.
Gastroenteritis is highly infectious, very easily spread from person-to-person and people are contagious for up to 48 hours after symptoms have ceased. Very young and older people are especially vulnerable.
Medical officer of health Andrew Lindsay says people should avoid visiting rest homes and early childhood education centres (ECEs) if they have symptoms.
Linda Ryan, the hospital's associate director of nursing, says it's really important not to visit people in hospital if you have diarrhoea or vomiting, and for up to 48 hours after those symptoms cease.
"We are managing the outbreak and are otherwise fully functional but have that important message for our community," she said.
Children with symptoms should be kept home from schools and ECEs, and adults should stay away from their workplace, while they are sick and for 48 hours after their last vomiting or diarrhoea episode.
The symptoms of gastroenteritis are nausea, vomiting and diarrhoea, stomach cramps, headache and low-grade fever, chills and muscle aches.
Most people with gastroenteritis can treat themselves at home and help prevent other family members catching it by following good hygiene. Staying at home also helps prevent the spread in the community.