Have a look at S. laciniatum (one of the so-called "Kangaroo Apples", because the leaf looks like a kangaroo paw!).

Thanks, shaxhome.

It would appear that all 3 spp have similar juvenile and mature foliage and are to be found in the same locations, i.e. SE Aus, from NSW through to Tas.

I guess I will have to log it as S. spp, for now, while I do a bit more digging.

All 3 would appear to be a relatively short lived plants, so would have been planted in the not too distant past. Hopefully I can locate someone at the gardens, who may have access to any database of what is/was planted.