John:
You should have had a B709A Import Form to bring a rifle barrel into Australia from anywhere overseas. If you have previously exported, eg for gunsmithing services, written permission to import must be obtained from the
Commonwealth Attorney-General’s Department.

Here is the link to the appropriate Australian Customs information sheet:
http://www.customs.gov.au/webdata/resources/files/ImportingFirearmPartsAccess.pdf

If Customs discovers a rifle barrel entering Australia without paperwork, and their check determines that a B709A has not been issued to the recipient, a very nasty letter threatening legal action will be sent to the intended recipient. If a B709A is forthcoming in short order, Customs will probably overlook the transgression and release the barrel, otherwise it is seized and presumably destroyed. In some cases an item may be sent back to the country of origin at the intended recipient's expense, but probably not in the case of a barrel.

FWIW I have always sent the original B709A (after making a few copies) to the overseas vendor to package up with the posted item and never had any trouble. Even had two barrels turn up in my PO Box by doing this: they had been "inspected" by Customs and simply put back into the post. Not sure if this would still happen today though.

Hope this info is useful.