Biosecurity Queensland has confirmed the two new positive test results.
The results follow a post-mortem on a companion horse who died at Proserpine in northern Queensland earlier this week, as well as an additional horse that became sick overnight at the Brisbane vet clinic which has been under quarantine since the scare began.
This takes the latest number of horses known to be infected with the rare virus since July to seven, with two known cases at Proserpine (Cannon Vale) and five at Redlands.
Dr Ron Glanville, Biosecurity Queensland chief veterinary officer, said the new results, while disappointing, are not unexpected.
"We know Hendra virus can spread through the close contact of infected horses, so it is not a surprise that another two animals that had close contact with the previous cases have also now contracted the disease," he said.
"For infection control and animal welfare reasons, Biosecurity Queensland will euthanize the latest horse to become infected at Redlands this afternoon, taking the number of horses to die from Hendra virus since July to six."