The mosquito-borne disease not previously seen in NSW is believed to have been contracted by residents in the Albury-Corowa area, prompting a warning to cover up and spray against bites.

NSW public health physician Katherine Todd issued the alert on Monday after a Border person was admitted to a Melbourne intensive care unit while infected with Japanese encephalitis.

Ten or more possible cases of the disease are being investigated in NSW.

"There have been a couple of suspected cases from the Albury-Corowa area, but in terms of the more broader geographical distribution of the cases I can't comment," Dr Todd said before adding more data was likely to emerge with NSW and Victoria liaising on spread.

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Japanese encephalitis is usually found in tropical conditions and has never been detected further south than Cape York in Australia.

"It's unclear exactly what's happened to lead it this far south, it could be something to do with climate conditions, changes in the ecosystem, different migration patterns of birds," Dr Todd said.

She could not say how the person in the ICU contracted Japanese encephalitis, but she warned it was important those spending time near waterways, such as the Murray River, protected themselves against mosquitoes.

"For a very small proportion of people, less than one per cent, it can cause a severe neurological illness that can have quite bad outcomes, that's why we're taking this so seriously and we're really trying to get the message out to people that this is out there and they really need to take measures to protect themselves," Dr Todd said.

The disease has been found at the Rivalea's Corowa among newborn piglets and greater protections for workers, such as vaccines, are to be offered.

The encephalitis can reach humans via mosquitoes who have bitten infected pigs.

A Rivalea spokesman said: "There is no threat to food safety and consumers can continue to enjoy pork products without any concern.

"(It) cannot be passed to humans through contact with pigs or pork."

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