While I was in Sydney for the 2000 Olympics, I did some research on the book industry in Australia. This article covers some of the information I gathered which hopefully will be helpful to US publishers.
While Australia has about the same land size as the US, they only have 19.1 million citizens. This population is generally excited to learn and enjoys reading books from the States. Add the 3.8 million New Zealanders and you could increase your potential market share by almost 23 million people—about 10% of the US market! Australian bookstores sell books that are similar to those here in US stores, but there are some clear differences.
Many books that are popular in the US are also famous down under, but the covers are different. Cover design is a bit more plain in Australia, and there are many more softcover books than hardcover books. There are also very few small and independent publishers. Most of the books from the States come from large US publishers, and many of those are from Penguin or their other imprints. There are two major bookstores in Australia: Dymocks is somewhat larger than Angus & Robertson, but both are substantial and have several stores throughout the country.
One of the bigger independents is Ariel Booksellers, which has created a nice niche in Sydney. Borders is slowly trying to enter the market, but its influence cannot compare to the Australian-owned stores. A quick note: Australians are fiercely loyal and patriotic. So given the opportunity, they will support a locally owned company over one from the US. There are no major wholesalers in Australia (as we have Ingram and Baker & Taylor in the US). Much of the distribution is handled by large publishers like Penguin, John Wiley, and Random House. While these publishers obviously carry their own titles primarily, they do distribute books from others. Another distributor is ADS, but they are considerably smaller than the three publishers.
I met with some of the big-wigs in the industry; most notable was Craig Scutella. He is the owner of Dymocks Main Store and the man in charge of dymocks.com.au, the primary bookselling outlet in the country. If you want to pitch your books to him, he prefers e-mail as the first form of contact. You can e-mail him at firstname.lastname@example.org. Craig attends BEA every year and knows the US publishing industry well. Tell him a bit about your titles, and if he's interested, he'll ask you to send him a review copy.
Angus & Robertson is a bit more structured- they prefer for you to send review copies along with promotional material and Australian marketing information to their main offices at: Angus & Robertson, Level 14, 379 Collins St., Melbourne, 3000, Australia.
The Ariel prefers a query letter first. You can send it directly to: Ariel Booksellers, 42 Oxford Street, Paddington, Sydney, 2021, Australia.
The Australian offices of the distributing publishers prefer a query letter first. You can mail them to: Penguin, 487 Mahroonelah Hwy, Ringwood, VIC, 3134, Australia; John Wiley, PO Box 1226, Milton, QLD, 4064, Australia; Random House, Private Bag 32 MDC, Knoxfield, VIC, 3180, Australia; ADS, PO Box 3520, Tuggerah, NSW, 2259, Australia.
Best of luck down under!