Nothing. Google isn't going to forego the entire Australian market or even an economy as large as the Eurozone. They'd just pay their x% tax and suck it up and consider it cost of doing business.
Google is a company, not a nation state and governments ought to stop kowtowing in front of companies. Reminds me of the story about the elephants and the rope
Australia has 25 million people.
That's about half the size of Spain. The GDP of Australia is roughly equal to the GDP of Spain.
The only thing they extracted were their manufacturing operations, GM and Ford still sell vehicles in Australia. They even have the Focus which Ford dropped for the American market! Unless Google and Facebook are going to close their offices but still provide service to Australia, that seems like an entirely different scenario.
https://www.goauto.com.au/news/cadillac/spied-cadillac-ct5-p...
https://www.goauto.com.au/news/holden/gm-has-lsquo-a-future-...
And then after that the companies will get involved in politics in those places so that it never happens again. This in turn increases the chance of regulatory capture and corruption.
Also, don't forget that increasing the cost of doing business will discourage others from trying. Oh, and there's also the US government too. They might just end up siding with Google and Facebook.
Sounds like a great reason to change the distribution of wealth and systematically reduce the power and influence of corporations.