Two mosquito-borne diseases have been detected in multiple locations in northern Victoria, with particularly high numbers in Mildura in the state's northwest.
The prevalence of the Murray Valley encephalitis (MVE) virus and West Nile/Kunjin virus in mosquito populations mean the risk of human cases the in coming weeks is high, authorities say.
"These results mean that the MVE virus is increasing in the mosquito population and indicate a significant risk to people in these areas, particularly Mildura," Acting Chief Health Officer Clare Looker said in a statement.
MVE is a rare but potentially serious disease that can cause permanent neurological disease or death, but is generally mild or not severe enough to cause symptoms.
Symptoms for the mild form of the disease include headaches, nausea and vomiting.
Mosquitoes spread the virus to humans from infected animals such as waterbirds including herons and egrets.
"Residents and people visiting Mildura are strongly advised to take measures to reduce their risk of mosquito bites," Dr Looker said.
The acting chief health officer said residents and visitors should cover up, use repellent and limit time spent outdoors.
Victoria's health department said it would provide updated health advice next week.