Not sure it matters.

It's a relatively uncontroversial ban, with public support in Aus because of mental health concerns, and key social media sites complying.

VPN's come with their own minimum age 18 T&C's. As do the credit and debit that are usually required somewhere along the line to pay for the services.

Historically, if it's awkward to circumvent most people tend to comply; which means in turn that minority that can figure out a way around it are unlikely to find many of their friends present. While for majority there's unlikely to be much of a draw or peer group pressure to circumvent.

I'm sure Aus gov will monitor, media will highlight problems etc, but would be surprised if it was not actually quit effective.