SYDNEY, (AFP) - Australian officials were on Saturday working to isolate potential victims after uncovering two more cases of the deadly horse-borne Hendra virus, which has erupted in a second state.

Spread to humans from horses, Hendra can lead to fatal respiratory illness and has killed four of the seven people who have contracted it in Australia since it was first documented in 1994.

A fresh outbreak was detected in northern Queensland state in June, with nine people undergoing tests after exposure to a sick animal.

Queensland authorities said they had discovered a second case, about 70 kilometres (40 miles) from the first farm, which had forced two horses to be put down. The outbreaks were not believed to be linked.

"Test results overnight have confirmed this as a case of Hendra virus infection," Biosecurity Queensland said.

"There are eight other horses on the property that are being monitored closely."

Officials said as many as six people may have been exposed to the infected animal.

A third case had also been identified in neighbouring New South Wales state, according to biosecurity officials there, who stressed it was unlikely to be linked to the Queensland outbreaks.

It is only the second time Hendra has been found in New South Wales.

"The horse had been in a paddock containing a fig tree, so it is likely that flying foxes were the source of infection," state health director Jeremy McAnulty said.

Nine people exposed to the sick horse were being tested for the killer virus, but McAnulty said they were at low to medium risk.

"All of the human infections that have occurred in the past have been linked to high level exposures to infected horses," he said.

Named after the Brisbane suburb in which is was first documented and believed unique to Australia, Hendra is believed to be carried by fruit bats (flying foxes) and spread via their urine and droppings.

The bats, which have no symptoms of disease, then pass the infection to horses, possibly via half-chewed fruit or other water or food they contaminate, and these animals then transmit it to humans.

Around 50 horses have died, or had to be put down, in 15 outbreaks of the virus since 1994.