Alert issued after Sydney woman diagnosed with measles

A Sydney woman has been diagnosed with the measles in what is the third locally-acquired case of the infection in NSW.

The woman, who according to NSW Health is in her twenties, had not travelled overseas prior to her illness and had no known connection to other measles sufferers, NSW Health said in a statement on Thursday.

She is believed to have been infectious from September 11 to 24 and visited locations in the inner west and Sydney's CBD, including two medical centres.

She also travelled by train from Marrickville into the CBD and back again while infectious, NSW Health said.

Health authorities are now seeking to identify people who were in contact with the woman, including those at the medical centres and her workplace.

"It should be a reminder for everyone to check that they are protected against measles, which is extremely infectious," Acting Communicable Diseases Health Director Christine Selvey said in a statement. 

"It is likely that other people have been exposed to this case or other unidentified measles cases in the community so everyone needs to be on the lookout for the early signs of measles."

The woman is the third recent case of locally acquired measles in Sydney not linked to a known measles case, Dr Selvey added.

Symptoms include fever, sore eyes and a cough followed by a red, blotchy rash that spreads from the head to the rest of the body.

The measles vaccine is free for anyone born during or after 1966 who doesn't have two documented doses of measles vaccine.