Health authorities are warning Sydneysiders and passengers who were on a Qantas flight from Bali to be on the lookout for measles symptoms after a woman and baby were diagnosed with the highly contagious condition.

Passengers on a flight from Bali to Australia have been warned to look out for symptoms of measles

The new cases bring the total number of measles diagnoses in NSW to 19 since Christmas.

The infant was too young to receive their routine measles vaccine and developed measles within days of returning home to Sydney from the Philippines, NSW Health reported on Monday.

The woman in her 20s developed a measles rash a few days after arriving from Bali, NSW Health reported.