A stolen critically endangered tortoise has been returned to an Australian zoo after being sold on the black market.

The buyer handed the animal in after he realised its rarity, police said.

Officers described the man as a good Samaritan for ensuring the tortoise's return, saying he lost money in the transaction.

Perth Zoo keepers believed the 10-year-old radiated tortoise stolen was taken from its enclosure last Tuesday.

It is not the first time a radiated tortoise had gone missing from the zoo - two tortoises were stolen from the same enclosure in 2011.

Native to the island of Madagascar, radiated tortoises are critically endangered due to habitat loss and poaching.