It is, of course, silly. And I blame the rum.
King George III looked a lot like Nigel Hawthorne and suffered from a disorder called porphyria, which sent him quite mad. During one of his bouts of sanity he managed to upset the American colonials, who then decided they could do without rule from England and fought for independence. Which they got.
This was a Good Thing: it stopped transportation of convicts there. Did you know 50,000 convicts were sent to the American colonies? The next yank who makes a joke about Australians being convicts will be told "And we didn't invent Budweiser."
Jails in England began to fill. Someone read a report from Lt James Cook about this very large, empty landmass on the edge of the South Pacific, and had the brilliant idea that it would make a great place to send the convicts. It was unpopulated, or at least unpopulated by Europeans, and it was a very, very long way away.
The new colony of soldiers and prisoners landed in 1788 and struggled on the shore of Port Jackson (the real name of Sydney Harbour). In 1805 a new Governor was appointed: Captain William Bligh, famous for losing a ship and crew in a mutiny.
During the next three years he governed New South Wales as though it were a ship. He was immensely successful at this. In fact, he was so successful the result was exactly the same as his previous ship the Bounty, except this time the entire country mutinied. Bligh was imprisoned for two years then shipped back to England. The colony was run during this period by the New South Wales Corps, under the command of Major Johnson and backed by rich land owners, one of whom was John Blaxland. The Corps were also known as The Rum Corps because they controlled the flow of alcohol into the colony. This rum was used as a unit of exchange since coinage was in very short supply.
In 1813 Blaxland's younger brother Gregory took two friends named Lawson and Wentworth, and spent two months finding a way over the mountain range west of Sydney, a distance you can drive now in 90 minutes. He applied for land on the other side of the Blue Mountains but the new Governor, Macquarrie, refused his application. Blaxland became so peeved he sold all his live stock, planted grapes, and made award-winning wines.
The wines which weren't so award winning he sold to the NSW Corps, who sold it to everyone else until Macquarrie stopped them by introducing new coins to the colony and devalued the rum.
I mention all of this so you know who the New South Wales Corps were. And now I can say this:
The New South Wales Corps had two uniforms: one for Summer and one for Winter. On the first of March they switched from Summer to Winter uniforms, and on the 1st September they switched from Winter to Summer uniforms. To a population who was largely illiterate and lived in a new country with no real seasonal variation, it was the only signs that Autumn or Spring had arrived.
I suppose this is why Australia gets in first and starts its seasons on the 1st of a month.