The news comes as the the first person to contract the disease at the clinic was re-admitted to hospital after his condition worsened.
The man had already spent a night in hospital for observation and was discharged.
Medical officer Dr Brad McCall said at the time: "The person was in good spirits and not exhibiting apparent symptoms."
However, he returned to hospital on Wednesday night after his condition deteriorated. It is understood he is suffering flu-like symptoms.
David Lovell, of Redlands Veterinary Clinic, where the outbreak occurred, told ABC Radio: "His condition has taken a turn for the worse as he's had to be readmitted to hospital, which is very, very disturbing and distressing for us here."
He told ABC that a woman staff member had developed symptoms yesterday and went to hospital. She has now tested positive for the disease.
Queensland Health is now reportedly testing and monitoring the health of 50 people for the disease who may have had contact with horses at the practive.
Hendra is carried by native fruit bats and the virus can spread to horses, particularly during the bats' breeding season. In a handful of cases it can jump from horses to humans and two people are known to have died as a result. The infection of the woman from the clinic is the fifth known case in people.
There are no known cases of human-to-human transmission.
The outbreak at Bayside involved three horses. One died from the infection and another was euthanized. The third is recovering.
The death of a horse in northern Queensland has also been confirmed as being caused by Hendra and up to a dozen people who had links to that horse are being monitoring.
Testing of people potentially exposed to the virus will continue for some time, with antibodies to the virus taking up to two weeks to appear.