Small spiny tree of Salicaceae family.
Location: Brisbane, Australia
Kei apples are native to Africa, where it is called Umkokola, Amaqokolo and other.
I would call it plum, not apple, as it has size and flesh as a plum, with a few small white seeds inside.
It is very rich with ascorbic acid (vitamin C), so the fruit is too acid eating out of hand, but I made a beautiful "Kei apples" jam!