NSW Health's Keira Glasgow said the warning has been prompted by an increase in gastroenteritis outbreaks at childcare centres across the state, and a high number of children under five seeking treatment at emergency departments.

"Nearly 100 childcare centres reported outbreaks of gastro in August, double the average for this time of year, and at least 820 children and 165 staff have fallen ill," she said. "The number of children seeking treatment for the highly-contagious infection at EDs has also risen above usual levels, with 609 children seeking medical attention in the last week."

She said the best defence was to wash hands thoroughly with soap and running water before handling and eating food, and always wash hands after using the toilet. Children in childcare or school who develop vomiting or diarrhoea should stay home for at least 48 hours after symptoms have stopped.

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