There are a few reasons why Australia Post might return a parcel to the sender:
customer did not collect their parcel
the name on the parcel was not known at the address or someone declined delivery
the item was damaged
If the parcel is intact and is returned to your pharmacy, it is most likely because it was not collected at the collection point after a certain number of days. In this case, contact the customer and discuss the option to send the parcel again or Click & Collect in-store.
Ask the Australia Post representative to confirm the reason for the return.