The state's emergency departments have grappled with more than 3300 cases. Nearly 800 of those were so serious they had to be admitted to hospital for treatment.
‘A number of the outbreaks have tested positive to the noro¬virus, which is highly contagious and known for causing explosive outbreaks on cruise ships.’
Children aged five and under and young adults aged between 17 and 34 have been the worst affected by the mysterious outbreak.
In the past three weeks there have been nearly 50 outbreaks of gastro across places such as childcare centres, aged care facilities and hospitals, which health authorities conceded was likely to "under-represent" the true extent of the problem.
A number of the outbreaks have tested positive to the noro¬virus, which is highly contagious and known for causing explosive outbreaks on cruise ships.
Dr Vicky Sheppeard, the NSW director of communicable diseases, said gastro numbers had exceeded influenza cases, despite flu season now moving towards its peak in mid-winter.
She said, "This year has also been unusual in that there have been a lot of otherwise presumably healthy, young adults presenting."
The number of young adults admitted to hospital is "significantly" above the five-year average.
"Certainly people should stay on the alert even though we are starting to see a slight trend down this week; there is still many thousands of cases out there," Dr Sheppeard said.
She urged people to wash their hands regularly and to stay away from work, schools or childcare centres for at least 24 hours if they became sick with gastro-like symptoms.
Those symptoms include nausea, vomiting, diarrhoea, fever, abdominal pain, headache and muscle aches.