A health alert has been issued after two more people caught measles in Sydney taking to nine the number of people who have acquired the infection in NSW since Christmas.

The latest cases involved two people aged between 20 and 55, NSW Health said in a statement on Thursday.

"We know many adults in this age group have only had one dose of measles vaccine," communicable diseases acting director Dr Christine Selvey said.

"Anyone unsure of their vaccination history should see their GP for another dose which is free of charge."

The number of recent cases in and around Sydney means people may have been exposed locally and could be developing symptoms, Dr Selvey said.