The zoo's chief veterinarian says the lions and tigers probably got COVID-19 from an asymptomatic zookeeper, and it spread to the big cat population - all nine of them.

He said the zoo had noticed the big cats were sick because the lions and tigers were barely getting up to move and not eating well.

"It came on a little bit insidious," chief veterinarian Donald Neiffer said on Friday.

"I believe last Thursday we had a single animal who was a little inappetent ... but it's not something that requires us to jump right on it."

However, he said within 48 hours other cats started developing signs of lethargy, dry coughs and sneezing.

"That's when things began getting quite interesting here and we started enacting other biosecurity measures."

After testing the cats and getting them on different COVID-19 treatments for the symptoms, they are now coming back around and should be fine soon.

"We are seeing an overall trend moving upwards. Based on how they're behaving today, the majority of them are eating more, moving around more. There's less discharge and coughing. I would hope that there's some significant improvement seen in these guys by Monday of next week," Neiffer said.

Australian Associated Press