Adelaide Zoo's remaining flamingo — believed to be the oldest in captivity and the only one left in Australia — has died, with zookeepers only now confirming that the much loved creature was female.

Chile the Chilean flamingo was euthanased at the zoo on Friday night after its health deteriorated to the point that "no additional medication could have improved her life", Zoos SA said.

The flamingo was aged in its 60s, well beyond its life expectancy, and was suffering a range of health conditions including arthritis.

Chief executive officer Elaine Bensted said keepers and staff were devastated by the loss of one of their most popular attractions, but knew it was the right decision.

"We'd been monitoring her health for quite some time … it was a very, very peaceful end," she said.

"But it's still obviously sad to say goodbye.

"Chile had been part of the zoo for a very long time after arriving in the early 70s."

Interstate visitors had flocked to Adelaide Zoo to see the flamingo in recent years after Greater, the zoo's male flamingo, died in 2014.

"She was very loved. She was the only flamingo in Australia," Ms Bensted said.

"Every time she was standing out in this nice sunny patch out the front of her pond with this crowd of visitors all taking photos, it really literally looked like she was just posing for the camera."

Only after its death could keepers confirm Chile was female. ( Zoos SA: Adrian Mann )

But Chile spent much of the past few years outside of its enclosure and away from the eyes of onlookers, as vets tried to improve its health and control the pain.

It was not until Chile died that Adelaide Zoo staff were able to confirm the bird was female.

"Flamingos have internal sex organs so we were never really quite sure," Ms Bensted said.

"Because she'd never laid any eggs, the sort of assumption was most likely she was male."

The Chilean flamingo is found in most parts of South America, inhabiting tropical and warm climates.

It is estimated there are around 300,000 individuals remaining in the wild.