Some links which might be useful:

http://www.qldrifle.com/traveling-with-firearms
Travelling with Firearms - Qld Rife Association - includes Exporting

http://www.customs.gov.au/site/page4381.asp
Prohibted and restricted exports - customs

http://www.defence.gov.au/deco/firearms.asp
Export Firearms and Related Goods

http://www.defence.gov.au/deco/firearmspermit.asp
Defence Export Permit for Firearms

http://www.customs.gov.au/site/page4372.asp
Firearms and Weapons - Aust Customs

http://www.customs.gov.au/faq/ProhibitedRestrictedGoods2.asp
Prohibited / Restricted Goods - Frequently asked questions
How can I import or export a firearm / gun / rifle / pistol?

These links may help. A commercial customs agent may be necessary to process the paperwork.