The Department of Primary Industries is urging horse owners to vaccinate against Hendra, after the deadly virus was detected at a property near Scone - Australia's horse capital - this week.
The unvaccinated mare, which has since been euthanised, suddenly developed neurological symptoms on Friday, three days after it was isolated in a yard.
The department (DPI) said it did not have figures for vaccination rates of horses in the Hunter.
A DPI spokesperson said on Thursday it was "imperative that horse owners take all steps they possibly can to reduce the chances of their horses becoming infected" with the bat-borne virus.
"Horse owners should discuss a vaccination strategy with their private veterinarian, as vaccination remains the most effective way of reducing the risk of Hendra virus infection in horses," the spokesperson said.
The case was the Hunter's first and was the furthest south the virus, which is potentially deadly to humans, has been found. There have been several infections on the north coast of NSW, with Kempsey being the previous most southern case.
James Archibald, a lifelong farmer who owns a property along at Gundy near Scone, said he believed a "refresher" on how to identify the symptoms of Hendra would be helpful for many landowners.
Mr Archibald, who has 100 horses on his property, was not the owner of the horse that contracted Hendra.
"Our concern is only for the handlers, who probably didn't think of it as Hendra as the symptoms are similar to other colds horses may have," he said.
"At this stage, I see it as bad luck for the owners and fingers crossed that's the end of it. They did great identifying it as Hendra."
The infected horse's owner contacted Hunter Local Land Services after the neurological symptoms appeared and a district veterinarian collected samples on Sunday.
The infection was confirmed by the State Veterinary Laboratory on Wednesday.
DPI says no other horses at the property have taken ill, but they will be monitored daily. NSW Health is conducting a risk assessment on people who were exposed to the horse.
Twenty-two horses have died from Hendra in NSW since the first case was confirmed in the state 13 years ago. No people have died from the virus in NSW.