Here’s a clear, helpful write-up answering the question “When is autumn in Australia?”:

If you’re planning a trip, moving to Australia, or just curious about its seasons, you’ll find that autumn in Australia looks very different from autumn in the Northern Hemisphere. In Australia, autumn officially runs from March 1 to May 31 by meteorological standards. Astronomically, it begins with the autumnal equinox (around March 20–21) and ends with the winter solstice (around June 21). However, most Australians and weather forecasts use the fixed March–May definition for simplicity.