Some of the smaller Australia Post outlets appear to be struggling a bit and they are bending the cubing rule to our advantage.
Cubing is supposed to be done on all items but if it is not square, or is odd shaped, then they sometimes skip the cubing process, which reduces costs to us.
Yet some of the other outlets will follow the letter of the law and tell you they have to cube it. They will tell you it is done to give an estimate of its size, but by cubing it they also increase their sales and income from us.
Find an Australia Post outlet that knows the rules and is flexible, and then stick with them. I've found it is useful to shop around. The Australia Post outlets are not all consistent, intentionally or not.
Also, sometimes they hold onto packages and the little messages are not sent to us at our homes to tell us to collect our packages. Some buyers ask where their parcel is, and the first thing to tell them is to ring their local post office, and other local post offices, as parcels go astray and are kept/stored at the wrong post office without anyone knowing it.