Biosecurity Queensland Chief Veterinarian Ron Glanville said one of the animals had died, while a second was ill, however tests were yet to be finalised.

A third horse diagnosed with the virus died last week.

"One was put down last night and a post mortem will be conducted, but it could be at least tomorrow before we get any results," Dr Glanville said.

It is believed a fourth horse at the same property died recently, which experts suspected may also have been the result of Hendra.

However, a decision was made not to exhume the horse's body for scientific tests.

The latest cases follow a separate outbreak of the rare and potentially fatal virus on Brisbane's bayside, where three of four horses which tested positive have so far died.

The disease has also been transmitted to two people who worked at the same Redlands Veterinary Clinic alongside the sick horses.

The pair, a male vet and female vet nurse, remain in hospital under observation.

So far, 50 people and 37 horses have been screened for the virus, with a second round of tests due this week.

Meanwhile, an investigation is underway to pin-point the source of the outbreaks, which authorities say are not linked.

"We are still quite confident that it has been contained to the (two) properties," Dr Glanville said.

"It was always very unlikely that it would spread outside the properties because it is not something that is spread through the air.

"Because of all the publicity surrounding this, we've been investigating horse deaths all over Queensland - pretty much every horse that looks sideways, actually.

"It is most likely that the first horse diagnosed on the 26th (of June) was what we call the index case - or the case where it originated - which was at Redlands...but we'll probably never know for sure."

That horse had been kept in the same paddock for at least a year, and had likely caught the virus from a bat.

Dr Glanville said traces of the virus were often found in the birthing fluid of bats, which may have dropped to the ground and been picked up by the horse.