How I managed to find it in a convenience store while holidaying in Melbourne I'll never quite understand (destiny?). They certainly aren't common plants south of the QLD/NSW border so finding it was a stroke of luck. I had no problem bringing it on the plane back (they wouldn't dare try to part me with it!) and now it is sitting in my garden waiting to be planted out:
All my heliconias have exceeded expectations, particularly rostrata. I haven't provided them any special treatment but they grow well all the same. As for the psittacorums, there is a lot of conflicting advice on where they will grow and which varieties are the more cool tolerant. I find keeping them quite dry over winter is all that is really needed to ensure they grow well in this climate.
Hopefully as people become more aware of them down here we will start to see more appear in local gardens!