Apparently Steam's having a massive sale. In the incredibly unlikely event this piece of news escaped your notice, yes, the Valve-owned online store is selling games at significant discounts. Shame that Steam is not the friendliest website to browse, which makes finding the really good deals harder than it should be. Fortunately, there's the internet.
There are really only two tools you need -- the browser extension Enhanced Steam (which we've mentioned previously) and SteamDB. As Daav Valentaten over at GG:3 points out, you could avoid Valve's official store almost entirely and just use SteamDB to locate the best deals for games you're interested in, or if you're not sure what you're in the mood for, apply filters to shrink the available choices to a manageable level.
Unfortunately, SteamDB doesn't support Australian dollars, but using the default USD should give you a pretty accurate value (excluding games from publishers that insist on regional pricing).
If you just want to dive head-first into the store, Enhanced Steam will make perusing its catalogue a less daunting experience by letting you know what games you already own, among other features. Given how easy it is to grab a game here and there during Steam's many sales, you might be surprised by how many titles your library actually has.