Why do the seasons in Australia start on the 1st day of the month as opposed to the 21st, everywhere else?

I've always wondered why in Australia the seasons start on the 1st of every third month as opposed to most other countries I know of that start on the 21st of every third month. Eg. Summer in Australia starts on the 1st of December. However winter in the Northern hemisphere doesn't begin until the 21st of December. Shouldn't summer begin on the 21st of December as well? Are there any other countries that do this as well? And if so why? Which is the correct date?