The South-Eastern Sydney Illawarra Health Service is warning residents to avoid all mosquito bites after some of the insects found in Sydney were shown to be carrying a rare disease.

The Barmah Forest virus can cause flu-like symptoms of fever and tiredness, as well as rashes and swelling of the joints.

Director of public health Professor Mark Ferson says the virus has no specific treatment.

"The virus is recognised as being transmitted by mosquitoes and most cases that have occurred over recent years have been people that have been bushwalking or doing activities like that out in bushland usually up and down the New South Wales coast," he said.