Heavy rainfall and flooding has created prime breeding conditions for mosquitoes in Victoria, prompting authorities to issue a warning about serious diseases the insects carry.

Increased breeding in inland and coastal areas means that while mosquito numbers are already high, they are expected to increase, according to the state's deputy chief health officer.

In an advisory issued on Monday, Associate Professor Deborah Friedman warned Victorians the best way they could reduce their risk of contracting a mosquito-borne disease was to avoid getting bitten, and remove breeding sites around their homes.

Victorians could expect more mosquitoes out and biting in the upcoming weeks, she said.

While not all of the insects carry diseases, some may transmit the Ross River virus, which can cause fever, chills, joint swelling, a rash and muscle aches among other symptoms.

The symptoms can persist for months, and can take up to 21 days to show up once a person has been exposed, according to Prof Friedman.

Mosquitoes can also transmit the Barmah Forest virus, with both it and the Ross River virus showing up most frequently in riverine and coastal areas. They are each endemic across Victoria.

The insects can also transmit the Japanese encephalitis virus, Kunjin virus, and Murray Valley encephalitis virus, which can each cause serious illness, the deputy chief health officer said.

Vaccines are not available for the mosquito-borne diseases - including the Ross River virus - except for Japanese encephalitis.

Along with covering up, Victorians should use mosquito repellents, limit their outdoor activity if there are mosquitoes, and make sure holiday accommodation has mosquito nets, the deputy chief health officer said.

Prof Friedman also warned clinicians to look out for the diseases in patients.