Biosecurity officials have confirmed that the first case was, in fact, on June 20, at the Mount Alford property.
Samples were taken at the time the horse died but only submitted to Biosecurity Queensland after another horse was confirmed positive on this property on July 1. The horse was euthanised the same day.
The third positive horse at Mt Alford was put down this afternoon after its health deteriorated.
Latest testing has returned negative results for the remaining seven horses at Mt Alford and three at a property in the Kerry area at Beaudesert.
Queensland Chief Veterinary Officer Dr Rick Symons said the infected Mt Alford horse had been off its feed and under close watch since early yesterday.
"These latest results have also confirmed Hendra virus infection was the cause of death of a horse on the Mt Alford property on 20 June 2011.
"The remaining seven horses are showing no signs of the infection at this stage and we will continue to monitor them."
Symons said there were four properties presently under quarantine in the state - three in the Beaudesert area and one at Mt Alford, with a total of 35 horses being monitored.
"There will be regular testing throughout July, when any infection will become apparent," he said.
"We continue to test suspect horse samples from other locations around Queensland, with as many as five samples a day coming in for assessment.
"This reflects the heightened awareness among vets and horse owners about the ever-present danger of Hendra virus infection."
He continued: "I cannot over-emphasise the need for anyone dealing with sick horses to take appropriate precautions."
Symons said biosecurity staff were maintaining a high-profile presence in local areas with visits to pony clubs and a mobile office at various locations around Beaudesert.
"Today there are mobile offices in Boonah and Jimboomba providing information to the community."
He encouraged people to attend tomorrow night's Hendra virus information session at the Beaudesert RSL.
Queensland Chief Health Officer Dr Jeanette Young said Queensland Health had not identified any other person potentially exposed to the virus as a result of this latest incident.
"The horse is on the same property and therefore all those potentially at risk have already been assessed, and blood testing undertaken as required," she said.
"Queensland Health will continue to provide any assistance, counselling, information, testing or treatment that may be required."