There are a number of US manufactures that make 300/221 uppers for the AR.
http://www.pac-nor.com/
http://www.olyarms.com/
http://www.model1sales.com/
http://www.ar15barrels.com/
If you can get lowers in Australia then the upper should something easier to get imported. Of course I know nothing of importing and exporting of gun parts but, you can always ask.