This debate has been going on for some time and I'm sure BTT will have all the details and rightfully so being a rural consumer where not all items are available locally; heck I think any non-capital city user in Oz could argue against this (I will do pro-bono work for a class action if anyone is interested )
Even the higher than $1000 import GST was debated at length but passed and imho rightfully so.
I have been following this closely and when I was a potential importer below $1000 on a regular basis this was very important to me, now that I am not that person but still a collector of high end items this continues to be an issue.

The basic reason for the Government not introducing an Act where anything that comes through International post being charged GST is very simple, the cost to administer this outweighs any profit (GST) gained. Yes Ladies and Gentlemen, Boys and Girls this is all about profit.
Charging companies like ebay on imported items because they know they have actual evidence they are coming from an overseas source is a much more profitable exercise and puts ALL the admin on them, hence $$$ for the Government.
(On a side note I hate that ebay, Apple, Google etc don't pay their fair share of taxes in Oz when they should)
GST is a state distributed tax which I can only surmise that it would be divided accordingly amongst the end users and perhaps using ebay's end addresses to work this out, but ebay still has to provide the distribution information. To be honest this is out of my knowledge and pay grade.

I can't blame ebay if they choose to geo-block Aussies from overseas ebay, it makes economic sense, they are getting the burden of having to work out the aforementioned. Three letters VPN.

My five cents.