The Virus Neutralisation Test (VNT) is considered the definitive test to determine whether an animal has had the virus in its system.
Biosecurity Queensland acting Chief Veterinary Officer Dr Rick Symons said national policy meant that the horse at the Cawarral property will be humanely euthanased.
"This is a national policy," he said.
"Horses can have the virus in their system and recover, but there is an ongoing risk of the virus remaining dormant and reappearing in the future and this presents a potential threat to human and horse health.
"This horse has been in isolation on a quarantined property and this result doesn't present any new risk to human or horse health during the current Hendra response.
"The timing of the euthanasia is yet to be confirmed. The owner has granted permission for the Australian Animal Health Laboratory to conduct a post mortem on the horse for research purposes.
"The Cawarral property will remain under quarantine until Biosecurity Queensland is completely confident there is no chance of any further infection."
Three horses have died at the property. The two that died on August 7 and 8 did so from a Hendra infection, testing has revealed. The cause of death of a horse that had died a fortnight earlier could not be determined because its body had been disposed of.