It's been New South Wales practice since colonial times - sometime in the 19th century - although it's spread throughout the country in more recent years.
It supposedly arose because of the dates colonial era soldiers were allowed to switch from winter uniform to summer. Soldiers in their heavy winter uniforms (this was in the day when the redcoat was still the standard uniform) started keeling over on parade in early December and so the army declared summer would begin on the 1st December rather than the 21st and from there, they simply divided the rest of the year into three month seasons.
Makes sense! During the summer, I envied the RAAF getting to wear stubbies while us Yanks had to always wear trousers.
BTW, I left in 1984, not 82. I’m gettin’ old!