A small Byron Bay fish is becoming world famous after being photographed swimming inside the bell of a jellyfish only slightly larger than itself.

Byron Bay photographer Tim Samuel spotted the fish while on a free-diving trip off the coast of the popular New South Wales town.

He first posted a photo to his Instagram account in December, saying "I found a fish inside a jellyfish!"

"He was trapped in there, but controlled where the jellyfish was moving."

Two weeks later he posted a second image, which a commenter described as "amazing and creepy and baffling", showing his dive partner Franny Plumridge in the background.

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The images came to global attention after Instagram account DiscoverOcean shared Samuel's original picture.

On Sunday, Samuel shared a third image, saying his phone had been "going crazy".

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On social media site Reddit, Samuel added that the fish "would have a difficult time swimming in a straight line".

"The jellyfish would knock him off course though, and every now and then it would get stuck swimming in circles," he said.

Several followers on Instagram and Reddit questioned whether the fish could survive, but Samuel said he "let nature take its course".

"It was a difficult decision though, I did think about freeing him," he added.

The fish could have been a juvenile trevally, which are known to hide among the stingers of some species of jellyfish, according to Associate Professor Ian Tibbets from the University of Queensland.

"It's difficult to tell whether disaster has just struck, or whether the fish is happy to be in there," Dr Tibbets told Australian Geographic.

"Although by the photographer's description of the fish swimming, my guess is that it is probably quite happy to be protected in there."

Dr Tibbett added that the jellyfish looked like a type of stinging jellyfish called a cubomedusan, a group which includes the box jellyfish.