Right, I guess I should stop lurking and say something on this one... I live on the mid-north coast of NSW and if you think it's hard finding a decent tandem in Sydney....
I have 3 tandems at the moment. 2 flat-bar, 26" wheel ones that I bought in Australia on eBay. I must say I was lucky though, decent tandems don't come up often, probably because there actually aren't many - and I look all the time. I think those who go to the effort of getting one in from overseas are not going to sell very often.
My other tandem is a Santana road tandem which I bought on eBay from the US. It cost quite a lot to import - the actually shipping across the ocean part wasn't too bad - but all the added charges at both ends add up. There was road freight, paperwork and handling, transfer to sea freight and all sorts of charges at this end. Despite all that that I'm very happy with it. There was no duty payable on the import of it but there is GST payable at importation!
My family (of 5) went on the NSW Big Ride in March on the 2 flat bar tandems + tag along. There were about 6 or 7 "full size" tandems there. 1 Trek, 1 Cannondale, 1 Co-Motion (drool) and some other lesser brands. There were about 3 or so bike friday tandems too. You can order a Trek or a Cannondale at a LBS dealer I think and it will get delivered eventually. KHS are around and I think you can order them in.
My conclusion is if you want a "real" road tandem though, you have to find a way to get one from the US. Next holiday? Or get it shipped ($$$).
I hope this doesnt put you off. A nice tandem is worth it and we need more of them out here!