can't put barra in, they can't handle the low temperatures.
There's an interesting thread on ausfish about a guy who has converted his pool into a fishtank. Although he lives in Brizzy he reckons it will still get too cold for the barra to survive, but he has bream, tailor and a flattie in there now.
Apparently you can convert some fish to fresh from salt, especially those that naturally occur in estuarine systems since they have to be able to handle fresh water as well.
Guess you could start off with some bream. When you catch them, put them in brackish water and over like 24-48 hrs slowly change this to fresh, then pop them in the dam. They would need food of course so you'd have to look at getting small baitfish in there as well.
Only thing to worry about really is introducing noxious species in there as you could be fined. Other than that any species that occur naturally in freshwater and brackish water in NSW should be just fine.. best thing is you don't have to buy them, you can just catch them.