A child visiting from overseas has become the third person diagnosed with the measles in less than a week in NSW.

NSW Health on Wednesday night said the child had presented to Royal North Shore Hospital after becoming infectious on Christmas Eve.

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The child arrived in Sydney a few days earlier from a country that has a large measles outbreak and visited several locations while infectious - including the Oakvale and Bimbadgen wineries in the Hunter Valley and Chatswood Medical Centre.

It's the third confirmed case in less than a week.

A visitor from the ACT was infectious while visiting the Central Coast and Sydney's northern suburbs between December 26 and 30.

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The person visited Thornleigh McDonald's, Jasmine Cafe at Umina Beach and made several visits to Deepwater Plaza in Woy Woy and Umina Beach Shopping Centre.

A young adult from Sydney was also diagnosed with measles on December 29 after returning from Thailand.

NSW Health's Vicky Sheppeard said anyone who visited the same locations during the Christmas-New Year period should watch for symptoms.

"Because of these three cases being in locations where there are many, many people ... there is a possibility of secondary cases arising," she told reporters on Thursday.

The time from exposure to the onset of symptoms is typically about 10 days but can be as long as 18 days.

Dr Sheppeard, the state's communicable diseases director, added: "At this time of year with so much travel happening to popular destinations where there are measles outbreaks it's a high probability that we'll see more travellers or returning tourists bringing in measles."

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

Symptoms 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.