It's a biiiiig job, and to have any hope at successfully transplanting a mature specimen you'll need a digger or some sort. Extraction of as big a root ball as possible is advisable, and little if any soil disturbance before, during and after transplanting is essential if you want a shot at succeeding. If you try and do it by hand, or with a root ball that's anything below a 2m radius from the 'trunk' or shallower that 2m deep, the plant's very likely to die. It may look okay for a year or two if you don't do this as they usually die slowly after a botched transplant.

I've transplanted many successfully in my time, but it isn't a job I'd recommend amatures undertaking. I'd seek professional advice from a local specialist nursery.

Good luck.

"If you try and do it by hand, or with a root ball that's anything below a 2m radius from the 'trunk' or shallower that 2m deep, the plant's very likely to die. It may look okay for a year or two if you don't do this as they usually die slowly after a botched transplant."

I don't doubt that this is the case as I have always heard it was difficult to transplant a blackboy but it's interesting that old specimens sold by nurseries are never in huge containers.