A lot of people, including myself, use them extensively in Melb CBD. Too many actually - they are always full, meaning that you have to knock on the back door to get your parcel (which goes completely against the idea of the parcel locker, but that's another gripe for another day).
But to restrict deliveries to only Aus Post./Star Track (who uses that anyway?) really means you can only use it for online shopping from Aus stores (who does that?) and from shipping options from intl. stores that are passed on to Aus Post when they arrive in Aus (who can be bothered checking if this is going to happen?).
I don't know if it's because accepting deliveries from other carriers was expensive/time consuming (I doubt it) or because they just want to preserve and their monopoly (more likely IMO).
Either war, I think this will go down as a colossal misstep by Aus Post in its bid to remain relevant, and open the door for the convenience-store style pick up services Romp mentioned earlier (I happen to know that at least two big couriers in Aus are thinking of forming an alliance in part to pursue such an idea).
Whoever is responsible for this decision is an idiot.