A large amount of acts that do well down under are from the northern hemisphere, so they don’t tend to release too much ‘summer’ music over Christmas - so usually we will just have in our charts what the rest of the world does.
But it is summer and festival season etc, so alot of our music consumed during the period outside of the Christmas songs is probably seen as summer related more than winter. Christmas songs themselves do well, but not near as well as say the UK when it comes to charts in December.