A norovirus outbreak in Central Otago over the past fortnight prompted a reminder from the SDHB to be vigilant over the Christmas period.
The virus had been identified in two large outbreaks including a Central Otago sporting event due to ''water contamination'' earlier this month, the SDHB said.
A second outbreak had been identified at a social gathering in Queenstown.
Organisers of the Lake Dunstan Rowing Regatta in Clyde confirmed over 20 people were struck down with the virus.
SDHB medical officer of health Dr Marion Poore said water contamination was a probable cause of the outbreak following the event.
''The water tested was contaminated with E.coli which is an indicator of additional contamination,'' Dr Poore said.
''It is difficult to detect norovirus in water but the individuals who were ill tested positive for norovirus which indicates it was in the water.''
Organiser Simon Smith disputed this, saying the virus was pre-existing and had already been going through the schools.
''A lot of people drank the water and were fine. I didn't drink the water and I got crook.
''We're a tight crew that train together and all use the same equipment.''
Dr Poore said the ''community-based'' nature of the outbreaks was unusual.
Norovirus outbreaks were more commonly associated with situations in which people were living or working closely together, such as schools, colleges or residential care facilities, Dr Poore said.
Although the acute illness was generally over within 24 to 36 hours, people with norovirus were infectious for at least three days after the symptoms stopped and on some occasions for up to two weeks.
A SDHB spokesman said the virus could survive on contaminated surfaces even after cleaning with some disinfectants.
