A woman who died from Murray Valley encephalitis was the first person to contract the disease in Victoria in almost five decades.
The woman, aged in her 60s, lived in the Buloke Shire in the state's northwest.
Health authorities believe she likely became infected there or at Swan Hill along the Murray River in early January.
Both areas were impacted by severe flooding in late 2022, which provided ideal breeding conditions for mosquitoes which carry the virus.
It was initially thought she had contracted Japanese encephalitis virus however that was ruled out after her death.
There is no vaccine against Murray Valley encephalitis but the virus is known to be circulating in mosquito populations in northern Victoria.
West Nile (Kunjin) virus has also been detected in the area and it's believed Japanese encephalitis is also circulating after one confirmed case earlier in the season.
"People should continue to be vigilant and take steps to avoid mosquito bites - especially in northern Victoria," the state's Chief Health Officer Brett Sutton said.
"Simple precautions include wearing long, loose-fitting, light-coloured clothing, using insect repellents, removing stagnant water around homes or properties, and avoiding the outdoors when mosquitoes are observed, especially at dusk and dawn."
It is the first time the virus has been detected in Victoria since an outbreak in 1974.
Most people with the disease do not have symptoms but about one per cent of cases will develop severe illness.
"Anyone with these symptoms should seek urgent medical care," Prof Sutton added.
"Serious illness can result in death or long-term neurological complications."
Victorians who spend time in regional areas are eligible for the Japanese encephalitis vaccine.
