I've only had a single package lost by Australia Post and that was ~20 years ago when they were using external contractors for Parcel Post.

The biggest AusPost 'event' is your parcel shuffling between Sunshine West, Chullora and Welshpool (all three are fully automated 'dark' Distribution centres that have effectively no storage space, so parcels get shuttled between those DC's as a type of 'storage in flight')