Warning for others ordering or using Australia Post express shipping during this time.
Being an OzBargainer I made an eBay plus order to save a few bucks off the Amazon price. The order was via express post and estimated to be delivered 14-16 December I had prepared to expect to receive the item maybe a few business days later with the Christmas rush. The package was received by AusPost on the 13th, hasn't been processed and now estimates delivery for 29th December. I know Prime is having delays too but everything I've ordered with next day or two day delivery has still come in a reasonable time.
TL;DR
If you want something delivered in a reasonable time frame atm don't choose AusPost express delivery.
There are too many different variables and factors for you to claim that. Could be an issue with the package itself, could be the address, the sorting facility, particular staff force, system issue, glitch, backlogs, whatever.
Issues happen with logistics all the time, they're massively complex. Not everyone will have the same good or bad experience.