Has anybody ever heard of a tree dying through someone digging through underground 240 Volts which then came into contact with a Eucalypts roots.

I have not heard of this actually happening, but I suppose it could. If the electric line was broken and the hot end came into contact with the roots, there could be an electrical charge applied that would damage the root cells. Most damage to plants from electricity is from heating as the electricity passes through the tissue, frying the cell contents. The extent of the damage would depend on the current and voltage, time of contact, and moisture conditions in the root tissues and the surrounding soils.

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Russ Carlson, RCA