A fourth confirmed case of measles in Sydney this month has sparked a fresh warning from health authorities who say it is "highly possible" more people will be infected.

Key points: Measles is one of the world's leading causes of death in children

Measles is one of the world's leading causes of death in children It is rare in Australia, however

It is rare in Australia, however There have been four reported cases in Sydney in the past three weeks

The virus is extremely contagious and can be transmitted just by being in the same room as someone who has it.

The four cases in Sydney were people who had spent significant time in the metropolitan area and the most recent patient, a male in his 20s, had spent time on public transport in the city's inner-west.

He was known to one of the earlier patients and had also visited shops and eateries at Leichardt, Pyrmont and Neutral Bay on December 23 and 24.

"It's one of the most contagious diseases. Fortunately most people are immunised," said NSW Health's acting director of communicable diseases, Dr Sean Tobin.

"If you are susceptible — that is out have not been vaccinated — the chances are very high that you will become infected.

"One case is an outbreak. It's not a virus that normally spreads in our community. It is concerning."

A NSW Health spokesman said the disease could take two weeks to incubate and it was "highly possible" there would be more cases.

"Given we've got four cases in the last three weeks it's highly possible there could be more given the mobility of the disease," he said.

You might not realise you are infected

Dr Tobin said people who were exposed to the measles patients would likely be showing symptoms of the virus if they had been infected.

They include fever, a cough and sore eyes and then a rash breaks out about three or four days later.

"Now is the time those people might be unwell and might not realise they've got measles," Dr Tobin said.

Measles is uncommon in Australia, and vaccinations are readily available. ( ABC News: Giulio Saggin )

He urged people to get vaccinated against the virus which can have serious complications.

"If you develop the symptoms of measles, seek medical advice, but make sure that you call ahead to your doctor or hospital emergency department so that you can be isolated from others to minimise the risk of infection," he said.

"The measles virus is highly contagious and is spread through the air through coughing or sneezing by someone who is unwell with the disease."

There is no specific treatment for measles, which is uncommon in Australia but remains one of the leading causes of death among children overseas.

According to the World Health Organisation, 134,200 people died from the virus last year.