The actress and her husband, John Easterling, were not in the house when the incident happened yesterday, said police in Jupiter Inlet Colony, north of Palm Beach.
A local newspaper called the incident a suicide but police refused to confirm this.
The man was neither a resident nor a member of the family.
The nearly 7,500-square-foot house, located on 35 acres by the coast, was built in 1975 and was bought by the couple in 2009 for USD 4.1 million.
Newton-John is famous for her role as Sandy in the 1978 movie "Grease," based on the Broadway hit musical. She is also a four-time Grammy winner.