Autumn in Australia #1.jpg
Autumn in Australia begins on March 1 and means the day begins to shorten as it cools towards winter.
In the northern hemisphere, March 20 or 21 is actually the spring equinox and marks the start of spring. In the southern hemisphere, this is the vernal equinox and should be the de facto start of autumn.
The Australian seasons have been simplified by starting each season on the first day of each season's starting month. So summer begins on December 1, autumn on March 1, winter on June 1 and spring on September 1.
Whatever the rationale behind how the seasons begin and end in Australia, simply think of the Australian autumn as the months of March, April and May.