IIRC it's because Paracetamol has low threshold for overdose, and is often blended in with other cold and flu medicines in Aus so it is easy to exceed daily dose if you are say, taking some conbination of paracetamol + Sudafed (paracetamol+decongestant) + drinking a Lemsip (lemony vitamin powder with more paracetamol).

It is only packet size restricted in supermarkets, you can still buy bulk packs from chemists.

Chemist warehouse has big signs up saying you'll be limited to buying a pack of 24 in Melbourne.