Kevin Darmody was fishing with friends by the Kennedy River in Rinyirru (Lakefield) National Park on Saturday afternoon when he disappeared.

Authorities believe the 65-year-old from the town of Laura was taken after moving to the water's edge.

Friends who were with him at the time did not see the attack, but reported hearing him yell, followed by a loud splash.

Bart Harrison, who was not with the group, told ABC News a friend told him about the frantic efforts to find Mr Darmody.

"One of the blokes that was with him just came running out waving his arms... saying, 'He's gone, he's gone'," Mr Harrison said.

"Nobody actually saw it happen - everyone was just fishing, doing their own thing, spread out along the bank."

Two crocodiles were shot dead by rangers upstream from where Mr Darmody vanished during the search.

Police believe the human remains found in the crocodiles, which measured 4.1m (13.4ft) and 2.8m (9.1ft), are those of the fisherman.

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Queensland Police said: "Human remains were located within two crocodiles euthanised by department of environment and science (DES) wildlife officers.

"Police believe the remains are those of the missing 65-year-old Laura man, pending the formal identification process.

"Officers believe the two crocodiles (4.1m and 2.8m) were involved in an incident with the 65-year-old and were located upstream from where he was last seen.

"Search and rescue operations in Lakefield have been discontinued at this time. Police will now prepare a report for the coroner."

Mr Darmody was last seen at Kennedy Bend, a campsite at the national park.

According to a government website, "large crocodiles are frequently seen" in the area.