It's complicated. They colonised an entire continent they call "Australia" a little over 200 years ago by genociding the natives. They let that stew in relative isolation and develop its own unique way of talking, and now when an Australian's native accent is attempted by those not born here it's extremely difficult for them due to said relative isolation.
Now, this sounds like I'm describing a real country, but it's a conspiracy somehow.
EDIT: Definitely downvoted for the genocide comment. Coloniser mad lol.