A haul of cocaine with an estimated street value of $US48 million ($A51.60 million) has washed ashore on a beach south of Tokyo, with authorities suspecting seafaring traffickers.

An elderly local resident found 80 kilograms of the drug inside backpacks that were tied together and reported it to police, Jiji Press and other media said.

The bags were outfitted with a buoy and illuminated markers and authorities suspect that they were lost by seafaring traffickers.

Police reportedly said the drugs had an estimated street value of 4.8 billion yen ($A51.60 million).