Two people have been diagnosed with measles in Sydney, leading authorities to warn to be on the lookout for symptoms - fever, red eyes and a runny nose.

An adult and a baby in Sydney have been diagnosed with measles, prompting health authorities to warn more people could be at risk.

The adult, who most likely acquired the highly contagious disease in Singapore, has since spent time in Parramatta, Wentworthville, Rouse Hill and Kings Langley, as well as Blacktown Hospital and medical centres in Castle Hill and Carlingford.

The baby, who is still too young for vaccinations, has since visited Balmain and Broadway.

NSW Health's Dr Vicky Sheppeard said anyone who has been in these locations must be alert to symptoms of measles in the coming weeks.

These include fever, red eyes and a runny nose and cough, followed some days later by a red and blotchy rash.

"These cases signal that a measles outbreak could commence in Sydney in the coming days," said Dr Sheppeard.

"Susceptible people, such as infants under 12 months of age, and older children and adults who have not received two doses of measles vaccine, should be on the lookout for symptoms."

Measles is highly contagious and is spread through coughing and sneezing.