The Australian winter is quite short and not very cold by world standards. (A third of the continent is within the tropics.) Depends where you are, though. Here in Sydney it can be cold enough early in the morning to need a heavy jumper but by lunch-time a flannel shirt is warm enough for me. We haven't had snow here since 1936, and even then it was only a single day. In Canberra we got frost overnight and I needed the heavy jumper all day. Winter is actually Sydney's sunniest season; it Adelaide it's the rainy one.
Christmas in mid-summer is great,and for the past couple of years the weather has been blessedly cool -- around 25 degrees C or so. Here's a nice website that tells you how we do it:
http://tww.id.au/c/christmas-form.html A growing number of people have "Christmas in July" parties, when it's cold enough to do the turkey and hot pudding thing. Yes, the ships still sell Christmas cards with holly and snow and stuff, but not in July.