This is the longest day of the year. It is the moment when the Southern Hemisphere is tilted most directly toward the sun. While the Northern Hemisphere is shivering through the first day of winter, Australians are celebrating the "official" start of their summer.
The answer depends on whether you follow the calendar or the astronomical calendar.
For weather forecasting and record-keeping, the Bureau of Meteorology (BoM) and the general public in Australia define summer as starting on December 1st .
Summer is peak fire season. Always check local fire danger ratings before traveling into rural areas.