Police have warned that people will be arrested if they try to keep any of the marijuana that has been washing up on Florida beaches.

Large bundles of the drug have been found on the shores of St Johns, Flagler and Volusia counties since Hurricane Florence began its approach and police have since arrested one man "attempting to steal marijuana".

Police received a call reporting a number of packages had washed ashore and that people, including Robert Kelley, had been seen "attempting to open the packaging and take packages of marijuana".

When police approached Mr Kelley he told them he was holding the marijuana for law enforcement arrival, but in a subsequent search of his vehicle police said they found a 5-kilogram "brick" of marijuana wrapped in plastic and concealed with a brown beach towel.

Tests revealed that the "green leafy substance" was marijuana and Mr Kelley was arrested.

Robert Kelley was arrested for possession of marijuana over 20 grams. ( Flagler County Sheriff's Office )

Police said that in two days they had recovered about 45 kilograms of marijuana that had washed ashore.

Police said the packages of marijuana had been turned over to US Customs and Border Patrol and that they would continue the investigation.

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Flagler Sheriff Rick Staly said this was "another great example of 'See Something, Say Something'".

"To anyone thinking they can take advantage of marijuana washing up on shore I have a warning for you," Mr Staly said.

"Is it worth a trip to the Green Roof Inn charged with a felony just for some free weed?"