I wouldn't dose with metronidazole if I wasn't 100% sure that my fish are suffering from flagellates. No need to get it of a vet BTW. Your own doctor can prescribe it. Brand name is Flagyl and there are numerous generics. You don't mention how much you dose so I cannot comment. Metronindazole is pretty hard on your filter bacteria so keep an eye on water parameters.
I don't know if Sera products are available in Australia but they have a new product called Sera med professional Flagellol. A lot of discus keepers swear by it .
Here's the link to the product information
http://www.sera.de/index.php?id=709&L=1