There has always been some confusion as to call a turtle a turtle or a tortoise. The international trend or accepted standard if you like is to call the terrestrial species tortoises and the aquatic species turtles. Some of the Australian freshwater turtles have previously been called tortoises but that really goes against the accepted standard and is now changing. Australia does not have any terrestrial tortoise species, only freshwater and marine turtles.
In Australia we can claim 6 species of marine turtles and about 20 species of freshwater turtles, I say about because the number is not constant. That is a lot, particularly for the marine turtles since there is only 7 or 8 species worldwide.
Freshwater turtles are mainly divided into two groups long and short necked turtles. The long-necked turtles are often referred to as side necked turtles because they crank their necks sideways to protect the head by the shell when threatened.