Where the topic gets interesting is in the subtext. Asking "when is fall" implies an expectation of what fall looks like. For travelers from North America or Europe, "fall" means crisp air, turning leaves, and pumpkin spice.
The answer to "When is it fall in Australia" is a perfect example of how geography dictates lifestyle. It is a season of "second summer"—a glorious, gentle tapering off of the heat rather than a sharp drop into winter. It is highly recommended for travelers looking for a "shoulder season" trip, provided they don't pack heavy coats until at least late April.
Just don’t expect to find Halloween decorations on every corner. In Australia, autumn is less about spooky season and more about —grape picking, long bushwalks, and the quiet, beautiful slide into winter. 🍂