The Earth tilts as it goes around the Sun. That tilt is what gives us seasons. While the northern half of the world is tilted away from the Sun in December (winter), the southern half is tilted towards it (summer). So a December day in Sydney can be 30掳C while a December day in London is 5掳C.
An Aussie Christmas Day might start with presents in the morning and then a swim. Many families spend the day at the beach, with a barbecue lunch of prawns, sausages, salad and fresh fruit instead of a roast turkey. Some still cook the turkey because that's the tradition - even if it's 35掳C outside.
Aussies still hang up Christmas trees, sing carols, and put fake snow on shop windows - because Christmas pictures and films are usually made in the northern half of the world, and people grow up loving those snowy scenes. There are also Australian versions of carols: 'Six White Boomers' (about kangaroos pulling Santa's sleigh) is a real song.
Father Christmas has to adapt too. In Australian shopping centres, Santa is sometimes spotted wearing shorts under his big red coat - and looking rather warm. Some beaches even have a swimming Santa in board shorts and a Santa hat.