The 21-year-old tiger was put down on Saturday due to ailing health, the zoo said on its Facebook page on Monday.

"As an elderly tiger he had begun to show a decline in his appearance in recent times and every possible measure was taken to maintain his health, comfort and well-being," the zoo said.

"He remained mobile and alert right up to his last day but the decision was made to humanely euthanise him after his health declined rapidly overnight."

Senior wildlife keeper Renee Osterloh said Bakkar stayed in his jungle bungalow den right up until his last day.

"It's a sad time at the zoo. Unfortunately I think when working with animals we all know when the time's right and the time unfortunately was right," she said.

Bakkar born on December 30, 1994.

He lived at Dreamworld and the Western Plains Zoo in Dubbo for some time before spending his last 11 years at the National Zoo and Aquarium.

Ms Osterloh said Bakkar was well-known for his "chuff".

"Bakkar was renowned for his vocals and chuffing which is saying hello in tiger talk and that was really quite special," she said.

"You didn't have to be around him for long to actually build up a really good connection and relationship and bond with him."

The zoo said Bakkar enjoyed the company of people.

"He'd spend most of the days (even when it was cold) over near the viewing window where he could watch the general public, but as soon as people went into the bungalow he'd move there for the night," the zoo said.

"Although he had access to his den as well, he'd usually stay where the people were and many Jamala guests have described how lucky they feel to have had that close connection with him. He was a great ambassador for his species and helped make a significant contribution in raising awareness about the plight of tigers in the wild."

Bengal tigers can live up to 12 years in the wild and up to 20 years in captivity.

The zoo said Bakkar outlived his siblings by a long time.

Hundreds shared their personal stories of Bakkar on the zoo's Facebook page.

One woman shared a photo of herself in Jamala Lodge with her hand and the tiger's paw touching through the glass.

"He used to come say hi as we had morning tea in the meet a cheetah bungalow. Such a beautiful and engaging soul," one wrote.

"Bakkar was simply the most magnificent boy. A very special favourite for so many. He was much loved and well cared for, special thoughts to all the zoo staff and family. Sadly missed by all," another said.

Ms Osterloh said: "At the end of the day he was an incredible tiger and he'll be sorely missed, not only the keeping staff who cared for him but also for the visitors and the wider zoo community as well."