An exclusion zone has been established around a sea mine police say is buried in the ocean bed off Port Lincoln in South Australia.

A member of the public alerted authorities to the existence of the mine, about half a kilometre east of Boston Island off Eyre Peninsula.

Police said the mine was about 20 metres below the surface and part-buried in sand.

The Defence Department said the mine was unlikely to explode unless something made direct contact with it.

Police said there was not considered to be any hazard to shipping, and the Navy was considering what to do about the find.

Peter Wyschnja from a local heritage munitions museum said two German sea mines washed up in the general area during World War II and another blew a hole in the British freighter SS Hertford.

"These mines usually have got weighted bases and they usually sit just below the surface," he said.

"It's quite possible that the holding lanyard from the weight to the mine itself has rotted away and this thing has drifted off into the distance."

He said his own experience with unexploded ordnance suggested it would be a significant operation to deal with the mine.

"There is still a possibility that it could have energetic materials in it that could explode," Mr Wyschnja said.

"They make a fairly big bang and cause a lot of destruction."