Basically, turtles live in or near water whereas tortoises only go to water to drink. Our turtles have webbed feet. Australia has no native land tortoises. Australia has both marine (6 of the 7 species worldwide) and terrestrial or freshwater turtles (24 of the 49 species worldwide).
It is a bit confusing to see both of these names used in the literature to describe a freshwater turtle. Basically tortoises are land creatures that usually have a domed shell, thick legs and club-like feet with no webbing between the toes. They live on land, eat mostly vegetables and only need a small dish of water to drink and mostly eat vegetation.
A turtle is aquatic. Marine turtles have flippers and spend their life in the ocean only venturing onto the beach to lay eggs. Freshwater turtles need to be in the water to feed but spend many hours a day drying out on a log or by the water. They have webbed feet with long, sharp claws and soft skin that is easily cut on rough surfaces. This double life has probably contributed to the name confusion.