"We are taking the situation very seriously and are working closely with the property owners, neighbours and vets involved," said state Primary Industries Minister Tim Mulherin.
Testing confirmed the death of an Anglo-Arab filly named Jackowah's Regal Princess from J4S Equine Nursery at Cawarral, east of Rockhampton, owned by John and Christine Brady, on Saturday. She was being prepared for the upcoming Magic Millions performance sale.
Two other horses are known to have died in the past two weeks, but there is a possibility one died of old age and the other a snake bite.
Dr Rick Symons, the state's acting chief veterinary officer, said the first two deaths were on July 28 and August 7, and more information is being sought to determine if these cases are related.
"The horses all showed varying symptoms of sickness," he said.
Mulherin said officers from Biosecurity Queensland were working with the property to get more information on the horses and movement of any horses off the property.
He said Biosecurity Queensland received reports of the deceased horse via its emergency disease-watch hotline on Saturday at 10.30am
"Biosecurity inspectors moved quickly and were on the property by midday to undertake a post mortem on the horse and quarantine the property when Hendra virus was suspected.
"The neighbouring property has also been placed under quarantine as a precautionary measure because a horse from that property had nose to nose contact with the infected horse over the fence," he said.
The confirmed case follows an outbreak 11 months ago among horses at Brisbane's Redlands veterinary clinic which resulted in the death of vet Ben Cunneen. A vet nurse also was infected, but recovered.
Dr Symons said staff from Queensland Health had been notified so that they could advise people involved with the sick horses on personal health issues associated with the disease.
One Australian newspaper reports that up to 30 people may have been exposed to the sick animal.
It is possible for the virus, which is believed to reside in the native fruit-bat population, to be passed on to horses and then on to humans. There have been no known cases to human-to-human transmission.
"Biosecurity Queensland will now take blood samples from other horses on the property to test for Hendra," Dr Symons said.
"The horses will be monitored, and their temperatures taken daily over the next 10 days.
"In terms of the quarantine period, that is likely to be 28 days from the last confirmed case of Hendra virus.
"Biosecurity Queensland will trace any horse that has left the property since three weeks prior to July 28.
"There are about 30 horses under quarantine with a number of different owners," he said.
"Biosecurity Queensland is putting in place a veterinary contact who will liaise and provide expert advice to the horse owners.
"Information packages will be provided to these owners and further information for vets, local residents and the public will be available on the Queensland Primary Industries and Fisheries website," he said.
This latest confirmed case means there have now been 12 Hendra incidents since 1994 involving 42 horses, with four resulting cases in people. Half have proved fatal.