I've got a bunch of Iranian Rials left over from a recent trip because I assumed, stupidly, that like other airports there'd be an exchange place after passport control. Sadly, that was not the case.
As a result, I'm stuck with a bit of IRR left over. Travelex won't take it, and the couple of banks I've tried won't either.
I asked a similar question about London a few years back, with some success, so I'm hoping for similar results with Sydney...
It depends what you mean by unusual. If you specifically mean Iranian Rials, I think you're out of luck. There doesn't appear to be any reputable money changers that will change them in Sydney, though I'm not excluding the prospect of finding some small and/or under the table money exchange business handling them in some areas of Sydney with an Iranian ex pat population, e.g. Fairfield or elsewhere in western Sydney.
If you mean more generally, here's a quick list of currencies that are exchanged by at least one provider, noting I only reviewed the major providers. I'd consider plenty of these unusual but YMMV:
