It's not every day that you spot one of Australia's venomous snakes cruising underwater, hunting and catching a native fish among rocky terrain in an Adelaide river.

Key points: The footage was captured along the River Torrens in the Adelaide Hills on Wednesday

The footage was captured along the River Torrens in the Adelaide Hills on Wednesday The snake can be seen surfacing from under a rock with a fish in its mouth

The snake can be seen surfacing from under a rock with a fish in its mouth An expert says red-bellied black snakes are common in dams, creeks and waterways

Adelaide's Sean Haydon caught — on camera — a red-bellied black snake hunting for fish along Adelaide's River Torrens yesterday.

Mr Haydon said the footage was taken on Wednesday about 11:30am near the Cudlee Creek township, in the Adelaide Hills.

In the video, the snake can be seen manoeuvring around rocks in the river before surfacing from underneath a rock with a fish in its mouth.

The snake disappeared under the rocks after capturing the fish. ( Sean Haydon )

"I knew they eat fish but have never seen one searching under rocks for fish," he told the ABC.

"At the end the snake moves under a large rock. [We] didn't see it eating but moving away out of site. Excited was an understatement."

The footage was sent to Snake Catchers Adelaide, which posted it on social media yesterday afternoon.

"This shows just how good they are at hunting underwater," the post said.

"One of their main food sources are fish as these beautiful snakes are found along our creeks and waterways."

Expert says they can 'hold their breath for half an hour'

Roly Burrell from Snake Catchers Adelaide said the snake in the footage was medium-sized, and that it was common for the red-bellied black snake to be found in the water.

Mr Burrell said red-bellied black snakes were fairly docile. ( ABC Open contributor Karl Granzien )

"They hang around dams, creeks and waterways and their food source is mainly tadpoles, frogs, fish [and] yabbies," he told the ABC.

"They are sort of an aquatic snake anyway. The pygmy copperhead in the Adelaide Hills feeds on tadpoles and frogs through the creeks. The tiger snake does the same.

"The only snake that doesn't really is the eastern brown, but if they're hungry enough they will eat a fish."

Mr Burrell said the red-bellied black snake was a fairly docile reptile and he thought it had actually adapted to spend more time in the water.

"They can actually hold their breath underwater for about half an hour," he said.

"You can get them right up to 2 metres (in size) in the waterways and it's not uncommon to see five or six swimming underwater.

"They are slowly adapting to become just like our sea snakes that stay underwater all the time."