An Indonesian teenager who survived 49 days adrift at sea has spoken about his ordeal, saying he ran out of food within a week and survived on fish and seawater he strained through his clothing.

Aldi Novel Adilang, 18, had been employed to mind a floating wooden fishing trap, but it slipped its moorings and he found himself adrift.

He said that he turned on a lamp every time he saw another ship and could not remember how many vessels passed by "unaware of my ordeal".

The Indonesian Consulate in Osaka, Japan, said the teen was rescued by a Panamanian-flagged vessel off Guam on August 31 (local time), about 1,900 kilometres from his original location, and returned to Indonesia with officials earlier this month.

He had been employed since age 16 in the one of the world's loneliest jobs: lamp lighter on a rompong — a wooden fishing raft with a hut on top — moored about 125 kilometres off the coast of North Sulawesi.

The coastline was not visible from the fishing rafts and the numerous rompong were kilometres apart from one another, said Mr Adilang's mother, Net Kahiking.

Supplies were usually dropped off to the light keepers about once a week.

Aldi Novel Adilang is rescued. ( AP/Indonesian Consulate General in Osaka )

"I was on the raft for one month and 18 days. My food ran out after the first week," said Mr Adilang.

When it did not rain for days, he said he "had to soak my clothes in the sea, then I squeezed and drank the water."

The teen's father, Alfian Adilang, said the family was overjoyed at his return but angry with his employer.

The rafts are anchored with ropes and Mr Adilang said strong friction caused them to break.

"I thought I will never meet my parents again, so I just prayed every day," he said.

The MV Arpeggio, which rescued Mr Adilang off Guam, contacted the Indonesian mission in Osaka when it docked and officials collected him on September 6, the Osaka consulate said in a statement.

He returned to Indonesia on September 8.

AP