I have two adjacent DEMs with different projections (see attached) I suspect the difference will be minimal but has anyone have an idea to the scale of the difference? i.e. Should I be concerned?
They're the same. My guess is that the GDA_1994_Transverse_Mercator DEM is a GRID or originally came from a GRID/TIN. That file format used the ArcInfo Workstation coordinate system format. It doesn't support as much as the ArcGIS format does.