I agree that the Camera Exchange is good. Though since it moved out of the CBD several years ago I lost track of it as I was usually in Melbourne on business and could not get out to the 'burbs' very much.
There are also a couple of stores in the CBD that seem to have a good track record of high end equipment including some MF stuff from memory.
at 391 Little Burke St is tiny but I recall it usually having some good quality 2nd hand gear and I have bought a few lenses from them over the years (Nikon and Leica). Though I don't get to Melbourne as much these days and its been a few years since I have transacted business with them. Obviously I cant vouch for whether they have the Distagon you crave.
The other one that comes to mind is Michaels Camera Store
at the corner of Elizabeth and Londsdale Streets in the CBD. It used to have and may still have a reasonable selection of 2nd hand gear in their large window display fronting Londsdale St. Again I have no idea if they have what you are after but all of the above have web sites you can consult.
Good luck and enjoy. I live in Adelaide but I love Melbourne - its a gracious city with a distinct European feel and lots to photograph.
Here is a selection of Melbourne themed Flickr groups.