The shock Japanese encephalitis outbreak was much worse than first thought.
Blood testing by New South Wales health authorities has shown JEV infections were up to six times greater than originally diagnosed.
With another La Nina wet spring and summer in the offing, NSW is expanding free access to vaccines to residents living in high risk areas.
JEV can only be spread by the bite of an infected mosquito.
JEV has been responsible for the deaths of at least five people around Australia when it arrived earlier this year.
Victoria has also offered free vaccines and has launched its own blood test sweep to learn more about the impact of JEV.
NSW Health has found one in 11 people who took part in its survey showed evidence of previous infection.
A total of 1048 people from Griffith, Temora, Corowa, Balranald and Dubbo provided blood samples in the survey.
One in 11, or 80 in 917 people, showed antibodies, indicating past infection with JEV.
Only around one per cent of people infected with JEV experience symptoms.
Officially, 13 NSW people had been clinically diagnosed with JEV - two of those people died as a result.
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Deaths were also recorded in regional areas of Victoria, Queensland and South Australia
Overseas travellers were excluded from the NSW survey - all were estimated to have acquired the virus between mid-January and the end of February.
"The results of this serosurvey provide us with valuable insight into the prevalence of past JE infections in these communities after it was first detected earlier this year," NSW Health chief health officer Dr Kerry Chant said.
Vaccines are recommended for people who live or work in Albury, Balranald, Berrigan, Carrathool, Dubbo Regional, Edward River, Federation, Goulburn Mulwaree, Greater Hume, Griffith, Lockhart, Murray River, Temora, or Wentworth shire areas.
People who are aged 50 years or older and spend significant time outdoors (four hours per day) are advised to ask their GP for the vaccine.
"Vaccination is an important part of the public health response but, currently, global supply of JE vaccine is very limited," Dr Chant said.
"So we're urging people to protect themselves by avoiding mosquito bites altogether, particularly as we head into warmer months."
Volunteers are being sought on the other side of the Murray River with a study launched in the Moira Shire north of Shepparton.
Free vaccines have been rolled out in Mildura, Swan Hill, Gannawarra, Campaspe, Moira, Greater Shepparton, Indigo and Wodonga.
SA Health is already offering free JEV vaccines to groups identified for priority vaccination by the Communicable Diseases Network Australia.
Queensland has been prioritising vaccines for piggery and outdoor workers.
It has only recently been confirmed JEV landed in Australia a year before the alarm was sounded.