Australia's horse death toll from Hendra now stands at 15, with the latest casualty confirmed in New South Wales.
Ten horses have died in Queensland and five in New South in little more than five weeks. It is the worst run of outbreaks since the deadly bat-borne virus was isolated in 1994 near Brisbane.
News of the latest horse death follows reports this week of a case in a dog which was on a property where a horse had caught the infection.
The Department of Primary Industries in New South Wales said the latest horse case was on its north coast, near Mullumbimby.
"The property has been quarantined and movement restrictions are in place for all horses and companion animals, including cats and dogs," the state's chief veterinary officer, Ian Roth, said.
"There are eight other horses on the quarantined property, of which four are considered at risk.
"A horse in a separate paddock is showing signs of illness but has been initially tested negative for Hendra. The others are currently showing no signs of illness.
"There are three cats and two dogs on the property - all will be assessed in line with NSW DPI's policy for companion animals."
Roth said the horse was not noticeably ill on Saturday but found dead in the paddock on Sunday morning.
"A veterinarian took a swab from the dead horse and sent the sample to NSW DPI's Elizabeth Macarthur Agricultural Institute for laboratory analysis."
Results confirmed the Hendra virus on Wednesday night, he said.
Roth said the Mullumbimby area property was the fourth in the state affected by Hendra virus after properties at Wollongbar, Macksville and Lismore were quarantined earlier in the month.
"It is likely that flying fox [bats] in fig trees were again the source of infection," Roth said.