Many of our clients do use Australia Post as a delivery agent. If your clients need your goods to arrive by Christmas, you should send it now because of all the disruption. Even with express post, Australia Post will not commit to delivery estimates now.
What I suggest you use is this calculator here.
I did ring up Australia Post, and the following are the official cut-off dates for sending for Christmas. Of course, they are not committing to them:
Send Parcel Post by Saturday 12 December 2020.
Send Express Post by Saturday 19 December 2020.
For sending letters within Australia this Christmas, standard delivery timeframes are in effect.
Sending internationally: They are too late for Australia Post.
I would suggest that you set our software delivery options to reflect this.