The warning has been prompted by outbreaks at childcare centres across NSW and a high number of children aged under five seeking treatment for the highly-contagious infection.

The number of outbreaks at childcare centres reported to NSW Health is above average for this time of year, with 109 centres reporting that almost 1000 children have been affected since February. More than 200 staff have also reportedly fallen ill during these outbreaks.

The number of children seeking treatment for gastroenteritis at emergency departments has also risen above usual levels, with 559 children seeking medical attention in the past week.

Acting manager of enteric and zoonotic diseases at NSW Health, Keira Glasgow, said gastroenteritis was often spread by direct contact with an infected person. Viral gastroenteritis is usually caused by norovirus or rotavirus.

"It spreads easily between people if they 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."

Symptoms of gastroenteritis include nausea, vomiting, diarrhoea, fever, abdominal pain, headache and muscle aches. They can take up to three days to develop and usually last between one or two days, sometimes longer.

The main treatment for viral gastroenteritis is rest and drinking plenty of fluids. Most people recover without complications. But viral gastroenteritis can be serious for infants, people with suppressed immune systems and the elderly.

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

People whose work involves handling food, or looking after children, the elderly or patients, should not return to work until 48 hours after symptoms have stopped to avoid spread of infection to others.

People who are not in school or childcare, do not prepare food for others or do not look after children or the elderly should stay home for at least 24 hours after symptoms cease.

Anyone recovering from gastroenteritis should avoid visiting hospitals and aged care facilities to avoid spreading the infection to those most vulnerable.

People with symptoms of flu and also urged to stay away from aged care facilities after a series of influenza outbreaks.

There were 2244 influenza cases confirmed in NSW in the four weeks to 3 March - almost double the 1144 notifications in February last year, NSW Health's latest Influenza Surveillance Report shows.

Eight outbreaks at aged care facilities were reported in February.

"Residents of aged care facilities will have little immunity left from last year's flu vaccine, so it's important to not expose them to the risk of influenza," branch director of communicable diseases, Vicky Sheppeard, said.

"If you have a fever, cough or runny nose please postpone visiting elderly relatives until you recover."

Publicly funded vaccines for high risk people for the 2019 season will be available across Australia from mid-April.