Heavy flooding in Victoria last year has created ideal conditions for mosquito breeding, leading to an outbreak of Ross River Virus across the state.

Victoria is experiencing a Ross River Virus outbreak following last year's floods, with the potentially-serious mosquito-borne disease now making its way into metropolitan Melbourne.

The state's chief health officer, Charles Guest, says 857 cases have been detected across Victoria so far this year, up from 20 to 50 in the usual year-to-date period.

Six of those cases have been detected in Melbourne.

Professor Guest says while many people may become infected with the illness, not all show symptoms, which usually appear between two to 14 days following a bite.

He says symptoms include fever, swelling of the hands and feet, headache and fatigue, and in rare cases can go on for months.

He told reporters on Thursday the virus is rarely, if ever, fatal.

Health Minister Jill Hennessy says extensive flooding in the state's northeast and northwest last year led to "ideal conditions" for mosquito breeding.

She says 18 councils across Victoria are working hard to reduce numbers of the pest, with specialised spray equipment and vehicles.

She also says "sentinel chickens" are being used for disease surveillance, a process whereby the blood of chickens in an area is tested regularly for antibodies to determine where breakouts occur.

Victorians are urged to wear mosquito repellant, install insect screens, remove stagnant water from their properties and seek help from their GP or local hospital if they experience symptoms.