A spike in the number of gastro outbreaks in December has prompted NSW Health to issue a warning to the community to wash their hands and stay home if sick.

The NSW health department has issued a public warning after a spike in viral gastroenteritis cases at child care centres and hospital emergency departments across the state.

A total of 86 child care centres had reported outbreaks of the highly contagious infection to NSW Health from the beginning of December until Tuesday, when 20 would normally be reported for the whole month.

Almost 550 children and 140 staff fell ill during these outbreaks, NSW Health said in a statement on Tuesday.

Those seeking treatment for gastro at emergency departments has also risen above usual levels with 2557 people seeking medical attention in the last week.

Almost a quarter of them were children aged under five and 644 people were admitted for treatment.

NSW Health's Keira Glasgow said increases of viral gastro are unusual in the state during summer, with peaks normally happening during spring.

"With Christmas around the corner, it is particularly important to pay attention to hand washing to prevent the spread of infection," Ms Glasgow said in a statement on Tuesday.

"Young children often need special assistance to make sure they are washing their hands properly.

"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 thoroughly after using the toilet, changing nappies or assisting someone who has diarrhoea or vomiting."

Symptoms of gastro include nausea, vomiting, diarrhoea, fever, abdominal pain, headache and muscle aches, NSW Health advises.

These symptoms can take up to three days to develop, and they usually last between one or two days, and can sometimes last longer.

While most people recover from gastro without complications, the disease can be serious for infants, people with suppressed immune systems and the elderly, the department says.