Yoga teacher Rachael Rogan was travelling on a bus at Engadine this month when she glanced out the window and saw "a beautiful blue marked Sulphur-crested Cockatoo".
As there was no one else on the bus, the driver stopped and they both took photos.
Ms Rogan contacted the Leader to share the photos and see if there had been any other sightings.
A friend provided a photo of a similar blue coloured bird, taken at Peakhurst about three years ago.
However, these apparent wonders of nature are likely to be something more sinister, says Taronga Conservation Society Australia research scientist Dr John Martin.
"This is definitely not natural," Dr Martin said. "It would appear this bird has been marked by a member of the public.
"Occasionally cockatoos are seen as a nuisance because they cause damage to things like plants and structures.
"Potentially, a person nefariously marked this wild cockatoo with paint to deter it from returning."
Dr Martin said it was also possible, but unlikely, the bird came into contact with a painted surface.
Two brightly coloured cockatoos, photographed recently at Bundeena by Rochelle Luke, were also likely to have been deliberately painted, Dr Martin said.
"It's extremely unlikely it was the result of an accident," he said.
Wildlife photographer Greg Tannos, who posted the Bundeena photos on his Find My Australia Facebook page, recalled coming across a pink coloured cockatoo at Menai in 2017, which the Leader reported at the time.
"I'm still not sure why this is happening or who would do such a thing," Mr Tannos said.
In 2016, cockatoos which appeared to have been painted blue, yellow, green and pink, were spotted in parks, trees and backyards in the eastern suburbs.
People can report sightings of unique birds and their behaviour to Dr Martin's research project through the Big City Birds app.