Hi Jiggity:  i really don't know enough about the lockers - I saw this on the Aust Post website and assumed you would have to use their labels.    perhaps it needs clarifying by Aust Post?

Prepaid parcel: You can use one of our prepaid satchels or boxes, or pay for and print a postage label using MyPost Business.

https://auspost.com.au/parcels-mail/parcel-collection-delivery/247-parcel-lockers#tab3