Illawarra Shoalhaven Local Health District public health director Curtis Gregory said there'd been double the number of gastro outbreaks this year, compared to the same period in 2018.

Around 750 children under the age of five and childcare staff have been affected by the viral gastro cases, mostly caused by norovirus.

"It's not surprising that young children under the age of five are susceptible to this virus in childcare centres as norovirus is quite contagious and spreads quite quickly," he said.

"Young children are often in close contact, they're very interactive and touch the same toys.

"The best defence against the virus is careful hand washing and with kids this can be quite challenging."

Across the state, nearly 100 childcare centres reported gastro outbreaks in August according to NSW Health - with at least 820 children and 165 staff falling ill.

The number of children being taken to emergency departments for the infection has also risen above unusual levels, with more than 600 seeking medical attention in NSW in the last week.

"Symptoms include diarrhoea and vomiting as well as fever, nausea, headaches and lethargy," Mr Gregory said.

"The main treatment is rest and rehydration. However parents or carers should look at going to hospital if their young child has a sudden increase in temperature or their condition deteriorates really quickly."

Kids should be kept home from childcare, or school, for two days after symptoms have cleared.

"We understand parents are busy and it's challenging for them to take time off work but one sick child can infect several others," Mr Gregory said.

Meantime, once notified, the public health unit works with childcare centre to control infection.

"It's important the health unit is notified as quickly as possible - the earlier we come in and help centres, the better the outcomes for children and staff," Mr Gregory said.