Phil, I live in Melbourne, will that do? [:)]
I can't help much with antique shotguns, it's not something I have much interest in. I found a link to one place that claims to specialise in English shotguns, they're not too far from where I live and they claim they ship to the US regularly. [url]http://www.mcdonaldsgunshop.com.au/[/url]
As for shooting here in Melb, try getting in touch with these guys: [url]http://www.melbournegunclub.com.au/pages/mgcintroduction.html[/url], [url]http://home.vicnet.net.au/~metrogun/[/url], and [url]http://www.claytarget.com.au/[/url]. The first two are clubs, the third is a national association which will know of other clubs in the Melbourne area. I don't shoot regularly, and only around the area of my parents farm, which is about 250 miles from Melb.
Both clubs I linked are on the outskirts of town in the nearby hills. If you're not hiring a car, getting there might not be so easy (I ride a motorbike, I don't own a car).