It's feared three people contracted measles in NSW from someone who brought the disease into Australia from overseas.
NSW Health on Friday said a young woman in her 20s and two children likely contracted the disease from another reported case in the state.
Dr Shopna Bag, from the Western Sydney Public Health Unit, says the three hadn't been vaccinated against measles.
"Measles is highly contagious and is spread in the air through coughing or sneezing by someone who is unwell with the disease," Dr Bag said in a statement.
"These latest cases reinforce the importance of getting vaccinated to protect against the disease. A highly effective measles vaccine has been freely available for many years."
There have been 11 confirmed cases of measles in NSW in 2017 with the first eight sufferers all contracting the disease overseas.
Travellers have brought back measles from destinations including Indonesia, Thailand and Malaysia.
NSW Health says the woman in her 20s and the two children spent time in the Sydney suburbs of Auburn and Chester Hill while infectious.
Symptoms of measles include fever, sore eyes and a cough followed three or four days later by a red, blotchy rash spreading from the head and neck to the rest of the body.
