What? There are small American species? You must be referring to bait fish.
Seriously though I did a search for non-native freshwater fish in Australia and the only suitable species I found were sailfin mollies, American flagfish and mosquito fish, all the rest were a trout or salmon of some sort. I don't know their range in Australia or how easy they would be to find but I'm fairly certain you won't be able to find any of our game fish that the others have suggested.
Here is a link to the site I used.
http://www.fishbase.org/Country/CountryChecklist.php