When I have dug my hole for planting, do I need to back fill with new topsoil or the soil that has been removed?

Once you have dug your pit for planting (which should be at least 25% larger than the container size) you need to back fill the hole so that when the rootball of the tree is placed in the hole, the nursery level is flush with the soil level.

The remainder of the pit surrounding the rootball then also needs to be back filled.

The soil used for back filling largely depends upon the quality of the soil that has been removed.  If the soil removed is friable and fairly loamy, then it can be used for backfilling.  If the soil removed is clay based, it is strongly recommended to use a 50:50 part mix of removed soil and a good grade top soil.  This approach can also be adopted irrespective of whether or not clay is present, as this will ensure you are introducing a good grade substrate to give the tree a good start.