The results follow a post-mortem on a companion horse that died at Proserpine earlier this week, as well as an additional horse at a Redlands vet clinic that became sick overnight.

This takes the number of horses known to be infected with the rare virus since July to seven - with two known cases at Proserpine (Cannonvale) and five at Redlands.

Dr Ron Glanville, Biosecurity Queensland Chief Veterinary Officer, said the new results, while very 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," Dr Glanville said.

"For infection control and animal welfare reasons, Biosecurity Queensland will euthanise the latest horse to become infected at Redlands this afternoon, taking the number of horses to die from Hendra virus since July to six."