Only one plane? I couldn't pick just one plane!
I'd find a nice council owned airport somewhere in rural NSW (Moruya would be ideal), buy that, build a mega house/hangar on it for myself, build other hangars and foster a truly GA friendly atmosphere to encourage GA companies and pilots to use the airport. I'd do this by keeping lease costs to a minimum and providing proper, modern facilities for all aerodrome users and keeping the local community aerodrome/aviation friendly. There might even be a few airside cabins/houses available for short term lease/holiday rentals.
Then I'd buy something like a PC 12 for getting around locally, a nice Lear 45 for getting around Oceania/SE Asia and just fly first class if I wanted to go abroad (obviously flying the PC12 or Lear to the airport!).
There would also be a few fun planes, maybe a Stearman and a Skybolt, plus whatever caught my eye. There would definitely be some sort of warbird in there and a Catalina restoration would be a great way to waste some money.
Would any of that be profitable? Probably not, but if I had $30million to waste, I'd have fun doing it and god knows, GA needs a shot in the arm from someone.