Two unvaccinated children have been diagnosed with measles in Sydney after returning from a trip to Sri Lanka, health authorities say.

The children diagnosed on Tuesday bring the number of Sydney measles cases to eight in the past three weeks, prompting NSW Health to again warn people to be alert for symptoms of the highly infectious disease

The children - their ages undisclosed - were on Singapore Airlines flight SQ221 from Singapore, arriving in Sydney at 7.40am on Friday January 11, according to a NSW Health alert.

The children diagnosed on Tuesday bring the number of Sydney measles cases to eight in the past three weeks.

They were also on a train from Sydney International Airport via Central station, which arrived at Meadowbank at 9.30am the same day.