Footage shows a sheriff's helicopter crew warning paddle-boarders they are swimming next to 15 great white sharks off the southern Californian coast.

The Orange County Sheriff's Department spotted the school from the air near Dana Point on Wednesday, instructing the swimmers to "exit the water in a calm manner".

A shark advisory is in place after a 35-year-old woman was bitten by a shark off San Onofre State Beach on April 29.