If you are going to to use Australia Post as a delivery agent if you require your goods to arrive by Christmas, then you better send it now. Even with Express post would be the latest you could send it. When I rang up on behalf of my clients, I was told the bushfires would not be an issue, and then they referred me to this calculator here. 

Of course, the claimed delivery schedules and actual arrivals do not always agree.

I would suggest that you set our software delivery options to reflect this.