Simplify the following text for a Grade 20 native English-speaking person.
Original: Although all territories of Australia are considered to be fully integrated in its federative system , and the official status of an external territory does not differ largely from that of a mainland territory ( except in regards to immigration law ) , debate remains as to whether the external territories are integral parts of Australia , due to their not being part of Australia in 1901 , when its constituent states federated .
Simplified: Although all territories of Australia are considered to be fully integrated in its federative system , and the official status of an external territory does not differ largely from that of a mainland territory , debate remains as to whether the external territories are integral parts of Australia , due to their not being part of Australia in 1901 , when its constituent states federated .