I live in Melbourne so our seasons are the opposite to those in the UK, US, Canada, Asia and Europe. But I know that Winter starts on December 21 up there? Why not just December 1? And does it end on March 21? In Australia (inc. Melbourne and Sydney) all our seasons begin on the 1st, so summer is December 1 to February 28, Autumn is March 1 to May 31, Winter is June 1 to August 31 and Spring is September 1 to November 30. Why does Winter in the NH start on this date and do all seasons start on the 21st?
Seasons are defined by people and vary according to the what they define as a season. For example, the Viking "primstave" which was a graduated piece of wood working as a calendar, had the 14th of October as the first day of the winter. Today, many Norwegians will tell you that the first day of the winter is the day the first snow comes and that varies from place to place and year to year.
Meteorology can’t build statistics based on those vague definitions and, in the northern hemisphere, the winter is, by convention, define as the period between the winter solstice to the spring equinox. That is from roughly the 22th of December to the 22th of March.
What you call then the Australian seasons are also in the mouth of the people because I can assure you that for Australian meteorologists and climatologists, it is the same as in the northern hemisphere.
Remember that the gathering of weather information is the most ancient and true international cooperation. Even during the times of world wars, countries cooperated to share they observations. Today it is the World Meteorological Organization (WMO), a division of the UN, that is in charge of gathering all data and redistribute it to all member countries. When those countries share climatic tables referring to the "winter," they all use the same definition.