The fossil remains, named Umoonasaurus and Opallionectes, belonged to the Plesiosaurs group and swam in an outback sea 115 million years ago.
The group of dinosaurs included a "killer whale" type predator from the Jurassic period, according to Benjamin Kear, a palaeontologist from the University of Adelaide.
Kear's team studied 30 fossils, mainly from around the outback mining town of Coober Pedy in South Australia state.
He said the long-necked marine reptiles swam in the shallow water of an inland sea that once existed in central Australia.
Freezing polar water covered large parts of Australia 115 million years ago, when it was much closer to Antarctica.
'Masses of teeth'
The Umoonasaurus was about 2.4m long and had three crest-like ridges on its skull.
"Imagine a compact body with four flippers, a reasonably long neck, small head and short tail, much like a reptilian seal," Kear said on Wednesday.
The Opallionectes was much larger, at least 6m long, with masses of needle teeth used to trap small fish and squid.
Kear said most of the fossils found were of juvenile creatures, leading the scientists to believe that they had discovered a seasonal breeding ground for the reptiles.
The discovery has led to comparisons with the Loch Ness monster, a mysterious and unidentified animal which is said to live in Loch Ness, a large freshwater lake in Scotland.
However, most scientists and experts have dismissed the creature, often nicknamed "Nessie", a hoax.