More people in NSW had Japanese encephalitis than first thought, a NSW Health study shows.
Eighty regional residents in the 917-person sero-prevalence survey showed evidence of previous infection, the health department reported on Wednesday.
Only 13 people in NSW have been clinically diagnosed with Japanese encephalitis this year.
Two NSW residents have also died from the mosquito-borne disease.
"The results of this sero-survey provide us with valuable insight into the prevalence of past JE infections in these communities after it was first detected earlier this year," chief health officer Dr Kerry Chant said.
"Vaccination is an important part of the public health response but currently global supply of JE vaccine is very limited.
"So we're urging people to protect themselves by avoiding mosquito bites altogether, particularly as we head into warmer months."
The state's free vaccination program has been expanded so people 50 and over who live in 14 regional local government areas and spend significant time outdoors can get the jab.
Victoria made similar changes to its vaccine program last month, after the state recorded 12 Japanese encephalitis infections and one death.
The state's health department also reissued a health warning on Wednesday, noting anyone living, working or visiting the Murray River may be at increased risk of catching the disease.
Japanese encephalitis typically occurs in parts of southeast Asia but was detected across Australia in January and February.
It is spread to humans by mosquito bites and can cause a rare infection of the brain.
People are encouraged to use insect repellents and wear long, loose-fitting clothing outdoors, especially as warmer months approach.
Five Australians have died from the mosquito-borne disease this year, with 40 confirmed or probable cases reported across the country.
