From what I understand they're basically asking foreign companies, with a turnover of over $75k, to voluntarily register for, and collect, GST. Like what some companies do already, notably Apple.
In
this article it mentions that they anticipate getting hundreds of the largest companies on board. That article also mentions that they won't be checking packages so it sounds like your average small/medium foreign business that ships to Australia can pretty much ignore the GST. And good luck getting every foreign eBay seller signed up!
TLDR: your iTunes/Google/Netflix/Amazon purchases will have GST, but that's about it...