I have little to add to what Julian, Charlie, & David have already written here. In short, it is highly likely that your car is a genuine GT5 but, as Charlie writes above, short of confirmation from the factory, there can be no absolute certainty. If you PM me, we can discuss sending a request to Claudia Lodi who, although she no longer works at De Tomaso, has been known to very generously volunteer occasional help.
Just one point about the issue of whether this car is from Australia. While it is true that the Panteras on either side of yours in the registry, as ordered by VIN, are from Down Under, this should not be taken as determinative. There is only the loosest occasional correlation, and usually none at all, between VINs and where the cars were shipped by the factory. It should also be noted that all cars originally imported into Australia were, to the best of my knowledge, all right hand drive. They also all have a distinctive importation plate in the front trunk (see sample below). I would be very surprised if your Pantera had any connection to Australia.