An under-vaccinated person has contracted measles in western Sydney, bringing the total number of confirmed cases in the current outbreak to 17.
The patient visited Sydney's Powerhouse Museum and Darling Harbour on April 14 while they were infectious.
They also went to Liverpool Westfield over three days in mid-April, Blacktown Hospital on April 15 and the Casula Central Medical Practice and nearby Chemist Warehouse three days later.
The victim had received only one dose of the vaccine - rather than the required two - prompting NSW Health to warn people aged 20 to 50 to check their vaccination history.
"Don't assume you are covered unless you have written records of two doses," spokeswoman Dr Vicky Sheppeard said in a statement on Friday.
"It is perfectly safe to have the measles vaccine again if you are not sure whether you've had two doses of the vaccination in the past."
The vaccine is free to people in those age groups through a GP, she added.
Vaccine schedules shifted between 1966 and 1994 and authorities are concerned this has created confusion.
The new case brings to 23 the number of NSW patients who have contracted the highly contagious virus this year.
Sydney's western suburbs have some of the lowest rates of immunisation in Australia.
In Parramatta and the surrounding suburbs, nearly a quarter of one-year-olds are not fully vaccinated.
That contrasts with the inner-west suburbs of Newtown and Enmore, where four per cent are unprotected, data from the National Health Performance Authority shows.
Health Minister Brad Hazzard this week hit out at the parents of unvaccinated children for being "quite irresponsible".
Measles is spread though coughs and sneezes and anyone displaying symptoms, which include fever, red eyes and a blotchy rash, should limit their exposure to others until they are diagnosed by a GP.
