Victoria's chief health officer, Rosemary Lester, has urged doctors to look out for signs of the potentially fatal virus after three cases were linked to an RMIT graduation ceremony at Etihad Stadium on December 17, which attracted about 33,000 people.

Two of the three people diagnosed with the highly contagious disease were aged 11 and 17 and both were unvaccinated. It is unclear if the third person, a 35-year-old man, was immunised against the disease.

Dr Lester said two of the three were now in hospital.

"We are concerned that more people may have been infected from the event," she  said. "Measles has an incubation period of up to 18 days, so illness acquired from this event could still be coming through, and cases could still remain infectious for many days."

A spokeswoman for RMIT said about 33,000 people attended the graduation ceremony. The university has contacted about 6800 of them to advise them on the outbreak.

Measles usually begins with common cold symptoms such as fever, sore throat, red eyes and a cough.

Dr Lester said the characteristic measles rash usuallybegan three to seven days after the first symptoms, generally starting on the face and then spreading to the rest of the body.

"Anyone developing these symptoms is advised to ring ahead to their doctor or hospital and alert them that they have fever and a rash," she said.

"If you know you have been in contact with a measles case please alert your GP or hospital emergency department. The doctor or hospital will then be able to provide treatment in a way that minimises transmission."

Dr Lester said she hoped high rates of vaccination in Australia would protect people and ensure the outbreak would be small.

Measles is a highly infectious viral disease that can cause serious illness including inflammation of the brain, particularly in very young children and adults.

"People can develop pneumonia and other serious complications from the disease, and those with measles often need to be hospitalised," Dr Lester said.

The groups of people most at risk of catching measles are:

• Anyone who is unvaccinated.
• Adults between 35 and 49 years, as many in this age group did not receive measles vaccine.
• People at any age who are immunocompromised - having an impaired immune system - even if they have had measles or have been immunised. This includes people with diseases such as cancer and people who are undergoing cancer treatment or are on high-dose steroids.