Alan, first you need to find out if your Australian regs allow you to import seeds. Then, if you can find a retailer that will mail them to you, that's your best bet. A lot won't even export to Canada, let alone Oz because of the complications.
I did a quick check on this site where I buy most of my seed, and most of the petunias they sell are plants, already sold out for this year. http://www.burpee.com/product/...
The seeds they do sell are amazingly expensive - some are $6.95 for 20 seeds. If you add that to the mailing or shipping cost, those are going to be some expensive seeds!
You'd have to e-mail the retailers directly to ask if they ship internationally. Use the "Contact Us" tab on their sites.