Steam and Australia have always had an odd relationship, with pricing disparities being the thorniest issue. Pricing's not the problem today though, rather availability — it looks like the Valve Collection, which contains 25 titles from the company's own catalogue, is no longer available to Oz on the popular digital distribution service.
If you try and hit up the URL directly for the Valve Collection (or Valve Complete Pack), you'll just be redirected to the main Steam page. On the other hand, if you visit it via a proxy, the pack appears perfectly fine.
According to this Reddit thread, the pack was available as recently as yesterday. The comments in the thread speculate that because the pack includes the uncensored version of the Left 4 Dead, the entire collection has been removed from sale.
Which is a shame, as it includes Dota 2, Counter-Strike: Global Offensive, both Portals and every Half-Life released for $US49.99. Let us know if you picked the pack up while it was available!