The two new cases take the number of people infectious with the highly contagious disease in the state since Christmas to 19, NSW Health said on Monday.

In one case, a woman aged in her 20s developed the measles rash after arriving in Sydney from Bali on Qantas flight QF44 about 6.30am on February 21.

Health officials are advising those on the flight, in the Sydney international terminal and visitors to the Opera House later that day to remain on the lookout for measles signs and symptoms until March 16.

A Sydney baby - too young to receive their routine measles vaccine - also developed the disease after arriving home from the Philippines.

Macquarie Shopping Centre (February 26 and March 2), My Health Macquarie (March 1 and 2) and the Northern Beaches Hospital (March 3) are among the places the infant visited while infectious.

Those in the same places at the same times should look out for symptoms until March 21.

Measles symptoms include fever, sore eyes and a cough followed by a red, blotchy rash on the head and neck that spreads to the rest of the body.

"If you develop symptoms please call ahead to your GP so you do not wait in the waiting room with other patients," NSW Health communicable diseases branch director Vicky Sheppeard said in a statement on Monday.

Australia is currently at high risk of importing measles due to outbreaks of the disease in southeast Asia.

Australian Associated Press