There are some more possibilities.
If the aircraft is imported temporarily, then the GST is paid as a bond and will be refundable.
It may be possible for an aircraft to be owned by an overseas based entity (with a Australian listed as operator). In which case the company does not pay GST in the first place (think Singapore, Delaware (USA) or maybe Malaysia). You will pay GST if that company has such a tax.
If you buy it in an operating company name or set up a company to own it, then the company will get a GST credit when it acquires the aircraft. When you sell it, its sold as an export sale with no gst.
On sale there is a profit or loss based on the price you get compared with the depreciated value on your books. This may make the GST look small.