People who attended an emergency department over a nearly six-hour window are being warned to watch for measles symptoms after a baby was confirmed to have the disease.

The child was too young to have been vaccinated and recently returned from south Asia, where ongoing outbreaks have been reported in several countries.

NSW Health has warned that people might have been exposed at western Sydney's Blacktown Hospital emergency department between 9.30am and 3pm on Friday.

Measles symptoms include fever, runny nose, sore eyes and a cough that can appear between a week and 18 days after being exposed.

"It's important for people to stay vigilant if they've been exposed and if they develop symptoms, to please call ahead to their GP or emergency department to ensure they do not spend time in the waiting room with other patients," Western Sydney Health District's Catherine Bateman said on Wednesday.

Initial symptoms are usually followed by a red, blotchy rash spreading from the head to the rest of the body after three or four days, she added.

"This should be a reminder for everyone to check that they are protected against measles, which is very infectious," Dr Bateman said.

NSW Health Minister Ryan Park said the case was similar to others seen recently as it involved a child who was too young to be vaccinated and had returned from overseas.

"We're asking people who were at the location on that day to be on the lookout for symptoms," he said.

"By remaining vigilant we can protect each other and the community."

The combined measles-mumps-rubella vaccine is available on the national immunisation program for babies aged 12 to 18 months and can be available earlier for those travelling to high-risk areas.

It is also free in NSW for people born from 1966 onwards who have not been previously vaccinated.

NSW Health earlier in February warned about potential measles exposures on a school bus and the emergency department at Murwillumbah Hospital, in the Northern Rivers region.

It came after three more alerts for the highly contagious virus in NSW, Victoria and the ACT in January.