Found this hope it helps from the John Cann book
Tortoise or Turtle?
The names applied to freshwater Chelonians or Testudines differ between countries. Australia, which has no true land tortoises, finds it convenient to use the term tortoise, leaving turtle for the marine species. There is one exception to the rule in the form of the Pig-nose Turtle (Carettochelys Insculpta), a recent discovery in Australia which indeed warrants the use of the term freshwater turtle.
Often when the subject arises, Australian tortoises are called turtles. In some respects this is perhaps a better term. In many other countries the use of the term tortoise is restricted to the terrestrial, or land-dwelling species and, as a result, our "pet" tortoises are often deprived of the water which is essential for their survival.