A spike in the number of gastro outbreaks in NSW childcare centres in August has prompted the government to issue a warning to parents.

The NSW government has issued a warning to parents after close to 100 childcare centres reported outbreaks of gastro in August alone.

NSW Health on Tuesday urged parents and carers to keep their children at home if they are sick, after a rise in viral gastro cases saw 820 children and 165 staff fall ill in centres around the state.

"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 (one) week," NSW Health's Keira Glasgow said in a statement.

"Infants or children in childcare or school who develop vomiting or diarrhoea should stay at home for at least 48 hours after their symptoms have stopped."

Gastro is often spread by direct contact with infected people and spreads easily if people haven't carefully washed their hands after using the toilet or before handling food, Ms Glasgow said.

"The best defence is to wash your hands thoroughly with soap and running water for at least 10 seconds before handling and eating food, and always wash your hands after using the toilet, changing nappies or assisting someone who has diarrhoea or vomiting," she said.

Anyone recovering from gastroenteritis has been warned to avoid visiting hospitals and aged care facilities to stop the spread of the infection to those most vulnerable.