The only way that rat became a “species” is because it’s ancestors (probably sewer rats from Liverpool) were traveling in a boat that crashed into the reef. The rats abandoned the sinking ship and managed to find a home on the reef. They are a human caused species.
Nope Melomys are different to Rattus
Probably evolved there after their ancestors drifted over from the mainland
Habitat range of Grassland Melomys
Humans probably brought them to Australia, some thousands of years ago