I can confirm that the rules do apply to the sender. You have to declare on a package what’s inside. For businesses using the normal postal system this is generally done electronically and also printed on the package so they’ll just be intercepted and binned. We ship product to Australia frequently and this is how it’s done. Same with private carriers but slightly different process. It’s still clear what’s inside.
Even if they try to avoid the ban by not putting correct customs information it’ll quickly become apparent to Australian customs which packages to seize who will then work directly with the private carrier if necessary (though it is prohibitively expensive to ship to more remote locations in Australia with private carriers so unlikely they will be used).