My understanding is that Interceptor has not been manufactured by Novartis since 2011.
I would worry that you might be buying old product (possibly past expiration date), or it's fake. Of course, it's possible Novartis has an Australian plant, but if they did, I would expect Novartis would have been importing that product into the US through legitimate channels, given the demand for it, after they had to close their Nebraska plant (that happened with immiticide when the plant in the U.S. stopped producing it --the drug maker imported it from its Euro. plants, and vets could still order it through legitimate channels from the drug maker---which has not happened with Interceptor, as far as I know).
Also, it's only supposed to be sold with RX in the US, which would increase my suspicion that the product is fake.
I wouldn't do it, personally -- too many reasons to be suspicious. Maybe it's fine gray-market product, but the risk to your dog is too high. If it's harmless but ineffective, the cost to treat heartworm can exceed $1,000. Not worth it IMHO.