This is the 6873rd most frequent Russian word.
австралия
Australia (a country and continent).
The word 'Австралия' is used here as a subject and refers to the specific continent.
Австралия - удивительный континент.
Australia is an amazing continent.
The word 'Австралию' is the accusative case of 'Австралия', used to indicate the direction or destination of action, namely, moving to Australia.
Он мечтает переехать в Австралию.
He dreams of moving to Australia.
The word 'В Австралии' is the prepositional case with a preposition 'в' indicating location, meaning 'in Australia'.