Experts are investigating whether a mutant strain of the gastro-like rotavirus is responsible for a surge in NSW cases after outbreaks in aged care facilities and child care centres.
More than 1300 cases of the highly-contagious virus have been recorded by NSW Health this year, with 412 of these recorded last month - the worst outbreak since 2012.
Rotavirus is a common cause of gastroenteritis for babies and preschool children and is passed from person to person by touching contaminated hands or faeces.
The health department is exploring why the virus hit with such a vengeance this year, saying it may have mutated - making more people susceptible to infection.
The effectiveness of a vaccine against the virus is also being investigated.
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"Immunity from the vaccine wanes over several years, so that every few years there will be sufficient numbers of people who did not gain protection from the vaccine, or have lost protection, to allow an outbreak to occur," a spokesman for NSW Health told AAP in a statement on Monday.
Despite a surge in infant vaccinations against the virus, the vaccine is only 70 per cent effective in preventing infection, the spokesman said.
However, it has worked to reduce severe cases of rotavirus with hospitalisations for infant gastroenteritis dramatically reducing, according to NSW Health.
Symptoms of rotavirus include fever, diarrhoea, vomiting and abdominal cramps.
