However, the good news was the surviving female white rhino who had been showing symptoms had improved, and was now doing well following intense monitoring and supportive medications.

Yesterday the senior veterinarian at the zoo, Benn Bryant, conceded while the testing had not been able to pinpoint the cause, with all the results in hand the working group agreed they were confident the remaining white rhinos pose no health risk and quarantine restrictions could be eased.

He said the prognosis for the remaining female was now good, while the two male white rhinos have shown no symptoms and are still healthy.

No other animals at the zoo have experienced this illness, he said.

Mr Bryant said the zoo’s veterinary team had worked tirelessly during this time, leading the investigation and consulting with rhinoceros specialists in Africa and North America, specialist government virologists and government vet services as well as pathology laboratories and specialists.

As part of these enquiries, a specialist working group was established, bringing together the zoo’s own experts with the state’s most experienced veterinarians and pathologists including the Department of Primary Industries Chief Veterinary Officer to assist with the exhaustive investigation.

An array of tests for bacterial, environmental and viral causes including a virology culture were conducted, and while a great many potential causes of the illness were ruled out the results were still inconclusive.

“The working group is confident that every possible avenue has been explored and that the full resources available to the zoo and the Department of Primary Industries have been utilised,” Mr Bryant said.

He said staff at the zoo were coming to terms with the losses while focusing their attention on caring for the remaining animals.

It is likely that the white rhinos will return to their exhibit in a couple of months following some scheduled maintenance on this exhibit.