Health authorities say the measles outbreak in western Sydney has worsened, with 16 cases so far, and more likely.

Key points: Measles cases climb to 22 across NSW, with 16 in western Sydney

Measles cases climb to 22 across NSW, with 16 in western Sydney In latest case man presented to Blue Mountains hospital with symptoms, could have infected others

In latest case man presented to Blue Mountains hospital with symptoms, could have infected others First major outbreak of measles in Sydney in five years triggered by returning traveller from Bali

It brings the number of measles cases across New South Wales to 22 so far this year.

In the latest case in the city's west, a man who was infected by an earlier case at Quakers Hill turned up at the Blue Mountains Hospital on Good Friday. He was isolated and is recovering.

Vicky Sheppeard, director of communicable diseases with NSW Health, said it was possible the patient infected others while spending time in Lawson, Strathfield and Leichhardt in the preceding days.

The patient visited Marion Aged Care Facility in Leichhardt on April 10, the Me Oi Vietnamese Restaurant in Strathfield that same day and went to the Lawson shops in the Blue Mountains again that day as well as on April 13.

"Unfortunately there probably still will be more cases to arise," Dr Sheppeard said.

"But the important thing about getting the message out is that if others do develop the symptoms, if they can isolate themselves rather than spread it further. That's the way that we are able to stop the outbreak."

The first major outbreak of measles in Sydney in five years was initially sparked by a traveller who contracted it in Bali, with it since spreading through western Sydney.

NSW Health told the ABC those infected have ranged in age from newborn babies through to 56 years of age.

Dr Sheppeard said the western Sydney outbreak included two cases related to Wyndham College in Quakers Hill and the others contracted it in Auburn and surrounding suburbs, either directly from the returned traveller from Bali or through others who it was passed onto.

"We haven't had a sustained outbreak of measles in New South Wales since 2012 so it is unusual to get so many cases following on, one after the other," she said.

"We did have a lot of introductions of measles from overseas this year and each time measles is introduced there is a risk that they'll come into contact with susceptible people.

"Once susceptible people catch measles in Australia then it can start to pass from person to person."

Various groups susceptible to measles

Dr Sheppeard said people were susceptible to measles for various reasons, including if they have not had it before or if they have not received the necessary two doses of vaccine to provide immunity.

Therefore, infants younger than 12 months of age are at risk as they have yet to be vaccinated, as are people in their 20s, 30s or 40s who may have missed out on one or two doses of measles vaccine earlier in life.

Those aged between 20-40, who may have missed out on one or two doses of vaccine, are susceptible. ( Reuters: Lucy Nicholson )

People who have problems with their immune system or who may have had cancer or chemotherapy are also susceptible to measles.

Dr Sheppeard said in the latest confirmed case of measles, anyone who might have been exposed when visiting the same places as the man did in Leichhardt, Strathfield or Lawson may develop symptoms within the next week or so as the incubation period was somewhere between seven and 18 days.

"There could also be other cases that arise from exposure in the Auburn area," she said.

"And the Hillsong Church is another place that people also went while infected."

She said Australia had "eliminated" measles but was yet to "eradicate" it.