As recently as 20 years ago Australians believed that turles only lived in the ocean, and that the freshwater turtles were in fact tortoises. Every old Aussie reptile book referred to them as tortoises by mistake as well. Even today in WA they still call them tortoises. Many people still have no idea that its been corrected, hense the arguements.
In a nut shell if it lives in water and has adaptations for swimming (web feet, flippers) its a turtle, either fresh or saltwater. If it has club feet and no swimming adaptations its a tortoise which is a land animal.
Australia only has native turtles, fresh and saltwater. Tortoises live overseas, but can be seen in zoos. Terapins is the American Indian's name for freshwater turtles.
Hope this helps