Zoos SA is investigating the unexpected deaths of seven female quokkas and two yellow-footed rock wallabies at Adelaide Zoo last month.
The zoo has run pathology and toxicology samples from the animals and is working with vets, horticulturalists and other experts to try and work out what happened.
In a statement, Zoos SA said at this stage "plant toxicity" was believed to be the most likely cause, but further testing was needed.
An 11-month-old joey called Marli survived and is being cared for by the zoo's animal health centre team.
Three male quokkas, who are also housed at Adelaide Zoo, are well but have been taken off exhibit.
The zoo said its kangaroo mob and the other 10 yellow-footed rock wallabies were "noticeably flat" after the deaths.
It said the animals were provided with supportive care and have made a full recovery.
"The welfare of the animals is our utmost priority, no other animals within the zoo showed signs of being unwell and the veterinary team are certain this was an isolated incident, contained within the zoo," the statement said.
"The loss of one animal, let alone a large group in a sudden incident such as this, is extremely upsetting, particularly for those who care for them.
"It reminds us of the importance of caring for all animals and our mission save species from extinction."
According to the zoo's website, more than 150 quokka joeys have been born at Adelaide Zoo over the years.
