They have their own tax systems. If other countries decide to retaliate by similarly charging their own VAT or whatever on low value Australian imports, then all consumers lose.
This is an expensive move by the Commonwealth, that all experts have recommended against. It will cost more to administer than it collects, it will reduce competition, and it will make it harder for Australia to compete internationally. It's a non-tariff tariff.
If the GST is to be collected in relation to personal imports valued at less than $1,000, then it should be collected from the recipient - as it is for imports valued at $1,000 or more! Of course, that wouldn't inconvenience international retailers and prompt them to consider dropping Australia from their customer lists. In other words, it wouldn't get Gerry and his ilk the desired outcome - because they are more than 10% away from competing with international competitors!
Australians pay too much for goods and services; this change simply writes that as a rule in stone by discouraging those competitors. It is totally covfefe!
Note: edited to remove reference to removed text.