Hi Michael, They may be planted but they've certainly spread since if it's the case (there's a couple young ones). They're basically next to the back fence of the dickson nursery so they could have come across from there as well.
Thanks Ned - most Spirea are from Japan or china and one has become weedy in America, though I think their weed potential in Canberra is pretty low - I will go to the effort of trying to identify to species level, given it seems to have spread beyond where it was planted.