At one time even iherb ltd were experiencing some trouble from our customs with the importation of b12 Methylcobalamin.
I have read here that some members were getting it through so i thought i would find out the facts, so i asked them myself, here is the gospel from Australian Customs.
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Thank you for your email regarding your wish to import Methylcobalamin from the United States for you personal use. I am from the Drug Safety and Evaluation Branch of the Therapeutic Goods Administration (TGA). The Drug Safety and Evaluation Branch is responsible for prescription medicines. In response to your enquiry, I offer you the following information.

Australian residents and visitors to Australia can legally import certain therapeutic goods for personal use under the personal import exemption that exists under the Therapeutic Goods Act 1989 and its associated regulations. This exemption does not allow the personal importation of either substances or drugs prohibited by Customs legislation, or, injectable drugs that contain material of human or animal origin (except insulin), unless an import permit has been obtained.

Methylcobalamin does not appear in the Customs (Prohibited Imports) Regulations 1956 and so will not require an import permit. Please note that it is illegal to supply goods imported under the personal import scheme exemption to persons outside the importer's immediate family. Under the personal import scheme exemption, you may import three months supply per import and, no more than fifteen months supply per twelve-month period. Three months supply is calculated from the maximum daily dose recommended by the manufacturer.