The first thing they could do is fine them. Google and FB might say "ha! but we don't have bank accounts in Australia!" and the government will simply collect from companies in Australia that have outstanding bills from Google for e.g. Adwords or Google Cloud.
Google can still run their websites to be reachable from Australia, but they wouldn't be able to do business in Australia. And what's the point of running an ad tech empire if you can't make money off of it?