Brisbane’s southside is on alert after a third measles case was reported, health authorities say.

A Brisbane man has contracted measles, becoming the third person diagnosed on the city’s southside this year.

Metro South Health says the man, who didn’t know he had measles at the time, travelled between Stradbroke Island and the mainland onboard a water taxi last Wednesday.

Authorities have been contacting people known to have been in close contact with the man.

Public health physician Dr Kari Jarvinen says residents need to be alert for symptoms of measles, particularly those who may have come into contact with the man.

“If you are unsure of your measles immunity or vaccination status, talk to your GP,” he said in a statement on Tuesday.

This year three people have been diagnosed with measles in the city’s southside.

Dr Jarvinen said measles was one of the most infectious of all communicable diseases and was spread through coughing and sneezing.

He said those with two recorded doses of measles-mumps-rubella (MMR) vaccine are very unlikely to get the disease.

Measles symptoms include fever, cough, a runny nose, then a red spotty rash and sore eyes a few days later.

About a third of those who contract measles are hospitalised.