"Samples were taken for Hendra virus testing and the positive result came back late last night," he said.
Symons said another horse was euthanised at the same property over a week ago and Biosecurity Queensland was seeking samples from that horse as well to test for the virus.
There are two remaining horses on the property.
Symons said Biosecurity Queensland was in the process of quarantining the property and would assess and monitor the other horses at the property over the next month.
Queensland's chief health officer, Dr Jeannette Young, said public health experts would visit the property today to determine how many people, if any, had contact with the infected horse.
"Queensland Health staff will continue to undertake contact tracing work to ensure all people potentially exposed to the sick horse have been identified," Young said.
Young confirmed that the follow-up results were negative for all 66 people being monitored from previous Hendra virus cases in Queensland in recent months.
Symons said this incident was the 10th in Queensland this year and this was the 12th horse to be infected with the virus.
"We have consistently said that Hendra virus infection can occur throughout the year and we remind horse owners not to become complacent, but remain vigilant for the signs of Hendra virus at all times," he said.
"We will deal with this latest case just as we have with previous cases through a process of quarantine, testing and monitoring.
"Each incident this year has been isolated and there has been no spread of the infection from one property to another.
"Our focus remains on learning from these experiences and pursuing a range of avenues of research using the $A12 million in funding provided by the Commonwealth, Queensland and New South Wales governments."