A fish that has never been seen off the east coast of Australia has been spotted on the Great Barrier Reef.

Marine biologist Jorja Gilmore was at the shallows of Opal Reef, about 48 kilometres off the coast of Port Douglas, when something in the water caught her eye about lunchtime on June 16.

"I was taking a guided snorkel group for a swim, when I looked down into one of the corals and saw something very unusual," she said.

Ms Gilmore had only been working on the reef since February, so she called master reef guide Tahn Miller over for look.

"I didn't know what it was, so I called over Tahn who had a camera," she said.

"I knew it was something from the deep ocean as it was very unusual, but I had no idea what it was."

Mr Miller said he had not seen anything like it during his 12 years working on the reef.

"The dorsal fins were at least a metre long and the body looked like the most polished silver you have ever seen, it was the colour of mercury," he said.

"I just knew that I had to record as much of it as I could."

Nobody onboard that day recognised the fish, so Mr Miller uploaded a screen shot to a network of 80 reef guides.

"Paul Groves who works on Magnetic Island suggested it was an oarfish, at the time I had never heard an oarfish," he said.

Adult oarfish can grow up to 8 metres in length, while fish that was spotted on the reef was only about 35 centimetres.

"Not very much is known about oarfish and it's thought that the adults live in very deep water," Mr Miller said

"There is so much that we don't about this species and there is a lot of misinformation on the internet."

Mr Miller reached out to oarfish expert Tyson Roberts, a former research associate of the Smithsonian Tropical Research Institute in Panama.

Dr Roberts said the fish filmed by Mr Miller was a juvenile Regalecus russelli, which featured a single dorsal fin crest with five to six extremely elongated rays.

"There have been other oarfish recorded in south-eastern Australia, but they are the Regalecus glesne species, which lives in cold water and has two dorsal fin crests above the head, differing from the individual spotted at Opal Reef."

He said the only other Australian record of Regalecus russelli was at Port Hedland in Western Australia.

Mr Miller said it was a once in a lifetime experience.

"There were about 40 people in the water that day and we were all very lucky to experience it," he said.

"It's amazing that the ocean still has secrets to reveal, just when you think you've seen it all, magic happens."