Tests will confirm the diagnosis, but Biosecurity Queensland spokesman Dr Ron Glanville said that the symptoms are similar to previous cases.
Dr Glanville told ABC News that the affected horse was put down yesterday, and another is under care. "They were very sick and likely to die, so the kindest thing is to put the horse down," he said.
A further round of testing of staff at a Brisbane veterinary clinic this week will determine if any more have been infected by Hendra.
Two staff have so far tested positive for the disease and are in the infectious diseases unit of Princess Alexandra Hospital.
Three horses at the clinic contracted the disease, with one newspaper suggesting a fourth horse was also confirmed as having Hendra there more than three weeks ago.
One horse died from the infection and another was euthanized.
The practice has been quarantined for about a fortnight.
Hendra virus, first isolated in 1994, is carried by native fruit bats and can jump to horses. Since 1994 there have have been five known cases of the virus jumping from horses to people, including the two currently hospitalised.
It is understood it can take up to a fortnight for infected individuals to return a positive test, so a round of negative tests this week may well ultimately see staff in the clear.
The 37 horses at the clinic are also to be tested this week to ensure no further animals have been affected by the disease.