Spoke to FinnAir CS, they are looking in to it.
There is Bus class availability on the SIN-SYD leg, but they seem to think the price would have been higher if I had been offered that originally.
Which is totally wrong. If you book a business class ticket you expect business class travel in all the legs of your journey.
Getting back to me about what they can do. The problem now is that a few weeks have past and the price has gone up if I cancel and rebook myself.