In an exclusive Californian suburb that has had only two murders in the past decade, the deaths of a billionaire's child and girlfriend at his mansion have caused a sensation.

Rebecca Zahau's naked body was found on Wednesday hanging from a rope tied to the second-floor balcony of the historic building in Coronado, a wealthy seaside suburb of about 24,000 people on San Diego Bay.

Her hands were tied behind her and her feet were bound, police said.

Just two days before, last Monday, she was babysitting Max, the six-year-old son of her billionaire boyfriend Jonah Shacknai, when the child reportedly fell down a flight of stairs at the 27-room home.