Simply withdrawing their business presence from Australia would not necessarily remove their liability or remove them from the jurisdiction of Australian courts. The lack of a business presence is not always a shield against long-arm jurisdiction.
As for resources on such treaties, visit the OECD website and you'll be inundated with the model treaties that form the basis of the actual treaties between countries. (Some treaties involve many parties, most are between two nations.) Since you're not paying me, I will simply point out that under America's tax treaty with Australia, the Technical Explanation would note that providing services to another country's citizens is an activity covered by the treaty, and Australia's tax office has plenty of documentation saying the same.
Australia would be more than welcome to do the same.
(But hee hee, we secretly know their citizens would protest if they set up a national firewall.)