"This is the first time outside of a laboratory that an animal other than a flying fox [native bat] or a horse, or a human, has been confirmed with Hendra virus infection," Symons said.
Symons said the dog was on a property where Hendra virus infection has been confirmed, one of eight properties in the state under quarantine.
"Biosecurity Queensland's policy is to test cats and dogs on properties where there are infected horses," Symons explained.
"In this case, the dog returned two negative results for the presence of the virus but a different type of test conducted at the laboratory has confirmed the presence of antibodies.
"This means that at some point the dog has been exposed to the virus but to our knowledge has shown no signs of illness."
Symons said this case raised many questions for biosecurity and health officials and researchers.
"We don't know how the dog contracted the virus or when it happened," he said.
"Based on our knowledge to date, it is most likely that the dog caught the virus from an infected horse.
"The remaining horses and dogs on this property are still being monitored daily and show no signs of illness.
"Biosecurity Queensland has tested other cats and dogs on the 11 properties currently under quarantine in Queensland and has received no other positive results."
Symons said Biosecurity Queensland recommended people keep dogs and cats away from sick horses to reduce the risk of such an infection.
Queensland's Chief Health Officer, Dr Jeannette Young, said Queensland Health would speak with the property owners to assess if there were any further people who may have had contact with the infected dog.
"We will continue to monitor the property owners and all previously identified contacts for infected horses on this property," she said.
"While we have not seen Hendra virus in a dog before, I believe there is a minimal risk of infection to humans from this animal.
"For a human to become infected, they would have had to have significant contact with bodily secretions - saliva and/or blood - that contain the Hendra virus."
Young said Queensland Health continued to have staff working on the Hendra response, including public health officials, medical and testing staff.