What if the platform is not registered as a business in Australia? You can't fine it if it's not a legal entity there. Simply setup a php Facebook clone and host it in another country.
"Simply" If you're making money from aussie customers, you need to comply to aussie rules
How will the Australian government compel such companies to comply?
Just by blocking the network traffic. Plenty of countries do this with sites they don’t like for various reasons.
Yes VPNs blah blah. But it will be pretty hard to operate some rogue social site when you can’t sell any respectable ads besides maybe porn sites and malware, and are only accessible via VPN. Pretty high barrier to adoption for a brand new site.