GPS was barely operational by 1990, so commercial interest would have been there, but probably not a great deal of exploitation.

Th 1994 shift must refer to the adoption of the Geocentric Datum of Australia 1994 (https://en.wikipedia.org/wiki/Geocentric_Datum_of_Australia_... ).

Different datums (models of the shape of Earth) will put the same coordinates at different places. The 656 feet would be the average shift required to reconcile coordinates using the previous datum with coordinates using the new one (which is based on WGS84, the modern global datum).