A rare moment has been caught on camera when a great white shark attacked another near the South Australian coast.

It was an ordinary day for Adam Malski, 33, who was sailing in the south Australian waters when a two-and-a-half-metre-long white shark swam up to his boat to feast on some bait.

Moments later, another white shark ripped through the first, sinking its teeth into the rival's head.

Malski was terrified at the reality of his inquisitiveness as he had asked a dive master just moments before the attack about the probability of a similar incident.

"The day where the shark attacked the other was surreal. I asked the dive master and skipper of the boat what would happen if a smaller shark got in the way of a larger shark," said Malski.

"The big shark would take the smaller one down," said the skipper.

"Literally six seconds later, that amazing scene was witnessed by me, the skipper and the dive master. They had never seen anything like it before. I believe that it is extremely rare footage as great whites are elusive and still largely unknown creatures," said Malski.

During a cage dive earlier in the day, Malski who is originally from London but now lives in Sydney spotted the larger shark.

The larger shark goes by the name of 'Gilbert' amongst locals, and was spotted in the waters around the Neptune Islands following the incident.