Co-ords do not shift...the datum shifts...I think you guys are mixing up going from AGD66 to AGD84 to GDA94. The first two are based on an Australian datum the last one is based on a world datum...the world datum is what the americans use for their GNSS. The only shift in position is/can only be due to seismic or tectonic movement. Like parts off the coast of NZ early this year...they are closer to AUs by about a foot so their co-ordinates have changed as referenced to a local feature.
As for surveying every five years...keeps surveyors in business but all they will found is a shift of 10cm or less....your threshold co-ords are going to move toward the north by less than a foot every five years...as said before magnetic north changes every year by an appreciable amount soooo the likes of any VOR station must be realigned regularly to ensure they are pointing toward magnetic north...Argument...my surveyed points from my seismic days are approximately 56cm north of where I surveyed them in 1984...However, my declared limit of accuracy was +/- 100cm both vertically and horizontally so my surveyed position is still good today if you were looking for oil....BUT the co-ordinates of that position have been shifted some 200m to the NE because of a change of datum.....hope that helps