A gorilla at Australia's Taronga Zoo in Sydney has been surprising crowds as he walks around upright on his hind legs.

Mahale, who is two and a half years old, is a Western Lowland gorilla.

According to Taronga Zoo's gorilla keeper Laura Fidler, his human-like behaviour is not entirely new.

"We are surprised at how often Mahale does it. He seems to do it all the time," Fidler said.

No one knows why he does it, but zoo officials think it is purely out of curiosity.

Western Lowland gorillas are endangered and Taronga Zoo is part of a global breeding program.

The World Wildlife Fund estimated that there are only 90,000 Western Lowlands gorillas in the wild, with numbers dwindling due to the destruction of their habitat, and the bush-meat trade in Africa.