I'm more worried about places like MD realising the burden of collecting GST on behalf of the Aus govt.
There's exchange rate to consider, the accounting and legal fees. Retailers will soon see that this doesn't make business sense, so they'll opt out of dealing with Australia, like Amazon US. MD might follow that path and I wouldn't blame them if they do.
Why don't the Aus govt put in place checks and collection of duties at the border like Canada and EU? Because it costs too much. Why bother with GST if it costs too much to enforce it? Because they want to protect local businesses. But what about us consumers?