A friend of mine contacted the TGA and the following info was given to her:
"Naltrexone is considered a prohibited import as set out in Item 13 of Schedule 8 of the Customs (Prohibited Imports) Regulations 1956. Therefore, naltrexone cannot be imported by an individual for personal use. Instead, an Australian-registered doctor must import the goods on your behalf. Please note that this cannot be done without an import permit being issued beforehand. This process should take place before goods arrive in Australia as the Australia Customs Service may not accept a permit that has been issued for goods after the seizure date.
If you require further information on this subject, may I direct you to our website. There is a TGA publication on personal importation which can be found at the following link. This link also contains information on Customs Prohibited Items under Appendix A and Appendix B. Naltrexone is included in Appendix A, item q "Preparations that purport to be a remedy for drunkenness, alcoholic habit or drug habit":