It's been 17 days since Australian backpacker Rye Hunt went missing, and authorities are "convinced" the 25-year-old drowned at sea after trying to escape from a deserted island in Rio de Janeiro.

No one quite knows exactly what happened to the Tasmanian on the night of May 22, when he is alleged to have mistakenly consumed the "highly potent hallucinogen and psychedelic" drug NBOMe, instead of MDMA.

But it looks like authorities are wrapping their search, with Brazilian police spokeswoman Ellen Souto confirming authorities were "well convinced" the Australian attempted to leave the uninhabited island of Cotonduba, located off the coast of Rio about 1km from Copacabana, but "failed" in his task.

Police have also returned Mr Hunt's belongings - found in his Copacabana apartment after he ditched travelling companion Mitch Sheppard at Galeao International Airport - to his family.

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Authorities have now moved on from discussing Mr Hunt's potential whereabouts to the morbid idea Mr Hunt died at sea, claiming the search proved "more difficult" as time went on "as the body starts to turn food into marine life".

It is alleged Mr Hunt inhaled the drug NBOMe, believing it was MDMA, before undergoing a psychotic episode and swimming to the island.

A Brazilian fisherman claims to have seen Rye on deserted Cotunduba Island, the day after he went missing. Photo / Facebook
A Brazilian fisherman claims to have seen Rye on deserted Cotunduba Island, the day after he went missing. Photo / Facebook

Brazilian police ceased the sea search for the missing Australian after 72 hours following reports of a body spotted floating at sea.

The fisherman, who wished to remain anonymous, told Rio's newspaper O Globo he had "no doubt" he saw Mr Hunt on Cotunduba Island, the day after his official disappearance on May 21.

According to the site Drug Info, "NBOMes can be in the form of blotting paper (similar to LSD) with images and logos from popular culture, clear liquid, white powder or a pill".

They "have a very bitter taste whereas LSD has no taste".

The drug, which is fairly new to the scene, is said to have come out of China.

Mr Hunt went missing on May 21 after he argued with his travelling companion, Mitchell Sheppard, at Galeao International Airport in Rio.

The fisherman is the first person to allege he saw Mr Hunt after his last official sighting; when CCTV cameras captured the Australian tourist leaving his hotel room in Copacabana at 6:20pm on May 21.

He said he first spotted the man, allegedly Mr Hunt, sitting on a rock. He eventually swum up to the Brazilian and his girlfriend and requested water. He was suffering "various injuries and seemed upset".

"The boy stared at my craft," he said.

"He had several cuts [to] the body: probably hurt [from] shellfish attached to the island's rocks. "My girlfriend and I were scared because he was a strong man. He went down near the water and made gestures asking for water. I asked him not to jump in the sea. I grabbed a 1.5 litre bottle of water. He drank it all at once."

"We talked very quickly in English, asked where he was from and how he got to the island. He said he was from Australia and had swum there.

"I found it strange, since Cotunduba is difficult to access, the waves enter with force and cover the island's rocks.

"When we stopped speaking, he returned to sitting, looking lost."

Yet searches of the area have proved hopeless.

There is sufficient evidence that Rye had consumed an illicit substance in the 24 hours leading up to his disappearance.

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Last week, a body with clothing similar to that worn by Mr Hunt was spotted by fishermen in the sea off the coast of Rio de Janeiro.

The sighting on Wednesday morning, near the island of Rasa, 8km from Copacabana Beach, was reported to the Brazilian navy.

Crew of a fishing vessel told authorities the body was seen near an archipelago about 8km from where Mr Hunt was last seen, but nothing has been found.

A Brazilian fisherman, who was allegedly the last person to see Mr Hunt alive retold, "I found it strange, since Cotunduba is difficult to access, the waves enter with force and cover the island's rocks".

The family has confirmed it will continue its search but has not commented on suggestions that Mr Hunt died at sea.

"There is sufficient evidence that Rye had consumed an illicit substance in the 24 hours leading up to his disappearance," Mr Hunt's family said in a statement overnight.

"Our initial understanding of what was consumed was a substance called MDMA. Reports from those around Rye in the lead up to his disappearance explained irregular and out of character behaviours, such as extreme paranoia, anxiety and fear.

"Based on these effects, we believe Rye may have unknowingly consumed a different illicit substance while thinking he was consuming MDMA."

It comes as police obtain DNA from Mr Hunt's uncle, Michael Wholdhan, who has joined the search in Rio, in the case a "body appears, the sample will be in the system".