Koolasuchus was an aquatic and estimated to have been around 4 to 5 metres (13 to 16 ft) in length. Its mass has been estimated to be up to 500 kilograms (1,100 lb).
Koolasuchus lived in rift valleys in southern Australia during the Early Cretaceous. During this time the area was below the Antarctic Circle, and temperatures were relatively cool for the Mesozoic. Koolasuchus likely lived in fast-moving streams.
As a large aquatic predator, Koolasuchus had a similar lifestyle to crocodilians. Although crocodilians were common during the Early Cretaceous, they were absent from southern Australia 120 million years ago, possibly because of the cold climate.