Eight patients are being kept in isolation at Wairau Hospital in Blenheim after an outbreak of a highly contagious illness.
The patients and at least 10 hospital staff have been hit with gastroenteritis.
Tests were being carried out to see if the outbreak could be norovirus.
Hospital Operations Manager and Assistant Director of Nursing Joyce Forsyth said that robust infection prevention practice is in place at the hospital to help prevent the spread of gastroenteritis to patients, staff and visitors.
She said that gastroenteritis is highly infectious, very easily spread from person to person and that it is very important not to come into the hospital - or any public place - if you know you are sick or think that you may have gastroenteritis.
Symptoms of the highly infectious illness include vomiting, diarrhoea and aching muscles.
The hospital said people should call Healthline on 0800 611 116 if they suspect they have it.