Shocking footage has emerged of the moment a Great White Shark broke into a diving cage while a tourist was inside.

The four-metre-long shark was chasing a piece of tuna bait left out by a guide when it accidentally smashed into what was meant to be a shark-proof cage.

One onlooker can heard to be expressing "oh my God" as desperate staff quickly open the top of the cage.

The shark appears to thrash around for several seconds as those watching ask "Is it inside?" 

Then the fish suddenly emerges from the water, clearly distressed and bleeding in several places. It manages to free itself and swims away.

Photographer and adventurer Buck Forester who posted the video said the incident happened in Guadalupe Island, Mexico.

He wrote: "The shark was biting at large chunks of tuna tied to a rope. When a great white shark lunges and bites something, it is temporarily blinded. They also cannot swim backwards. So this shark lunged at the bait, accidentally hit the side of the cage, was most likely confused and not able to swim backwards, it thrust forward and broke the metal rail of the cage."

He said the diver is a very experienced dive instructor, remained calm, and when the shark thrashed back outside the cage, the diver calmly swam back up and climbed out completely uninjured.