Brett Mann, 22, and his two friends had ridden their quad bikes 80km southwest of Darwin in the Northern Territory on Sunday when they stopped on the banks of the flooded Finniss River to wash mud off their bodies and bikes.
Mann was swept into the river by a strong current and as his two 19-year-old friends swam after him, a four metre crocodile grabbed him by the shoulder and dragged him underwater.
"They had gone to the river to cool off," police sergeant Garry Casey told reporters in Darwin.
"The river was in flood and a large tide was coming in and they probably did not take account of the tidal influence in that part of the river," Casey said.
"They were trying to make it to safe ground when they saw the crocodile in the reeds."
Casey said the crocodile returned a short time after attacking Mann and began stalking the other two men who had climbed a tree in the middle of the swollen river.
The pair was eventually plucked from the tree on Monday, 22 hours later, by a helicopter that had been searching for the trio after they were reported missing.
Northern Territory police said the Finniss River area where the attack took place was a notorious breeding ground for large saltwater crocodiles.
Police began searching the river on Tuesday for Mann's body
and the killer crocodile.