Lithium Polymer.
With Lithium Ion, the Lithium salt electrolyte is held in an organic substance, while in LiPo, the electrolyte is held in a polymer.
If damaged, they can explode, which I guess is the reason for the ban.
If shorted, they can explode.
If overcharged, they can explode.
If overdischarged, they can explode.
The power provided by them is a by-product of them not exploding, or perhaps them just exploding more slowly.
At the bottom of this page on Australia Post's website, there's a link to their Terms and Conditions. I can't see anything specific about LiPos.