A man in his 60s has contracted Japanese encephalitis in NSW, while Victorian authorities are reporting two suspected cases.
NSW Health says the man was treated in hospital after contracting the mosquito-borne virus but has since been discharged and is recovering in a rehabilitation facility.
It is the first confirmed case in the state's Goulburn area, with urgent investigations underway into where the exposure occurred.
Victoria's deputy chief health officer issued an update late on Friday to confirm an additional two probable cases of Japanese encephalitis there.
NSW is also testing several other people for the virus, with more cases expected to be confirmed over coming days and weeks.
The latest infections bring Australia's confirmed number of cases to 17: five in NSW, seven in Victoria, four in SA and one in Queensland.
Two of them have been fatal, one in NSW and another in Victoria.
The federal government on Friday announced a $69 million plan to combat the spread of the disease, including the acquisition of another 130,000 vaccine doses.
A $5 million public information and awareness campaign will be launched, while $3.5 million will assist in human testing.
To understand the spread, the government plans to enhance surveillance of mosquito and animal activities, model potential virus spread and undertake mosquito control.
Doherty Institute Infectious Diseases Physician Michelle Giles says more information is needed on how the virus is affecting the southern states and territories.
She says it's important to find out the true number of people becoming infected, not just those who are symptomatic.
"The majority of people will be asymptomatic or not know they've necessarily been infected or have mild symptoms," Dr Giles told AAP on Friday.
"If it's one in 200 people who are getting encephalitis, that means there's probably another few hundred people infected that you don't know about.
"That's really important work that needs to be done, to find out how many people might have been exposed without getting symptoms."
Japanese encephalitis is transmitted via mosquitoes and cannot be spread from person to person or by eating animal products.
About 99 per cent of cases are asymptomatic but some people may experience fever and headache, while one per cent could experience severe infection including convulsions, paralysis, neck stiffness, tremors and coma.
Children aged under five and the elderly are more likely to develop severe infection.
