"961 only went to SG after all the other class members had been withdrawn"
In case the original poster had missed this, 961 in its final form had lettering on the bogies reading "830 class traction motors fitted".
This concerned me a little, since 830s and 930s had always had the same traction motors (GE 761 or AEI 253).
However, I think this actually meant that 830 class axles and gear cases had been fitted, ie a set of axles and traction motors intended for the 830 class had been fitted, presumably because no spares existed for 930s after the decision to withdraw them.
What this meant was that the gear ratio was different, which would affect the speed at which the maximum continuous tractive effort would be achieved.
I'm not sure whether this contributed to it remaining stored in Goulburn or not. Certainly the owners (ARHS ACT) had other 44 class and 48 class locomotives and may have been concerned that some components could not be interchanged as desired.
It is a pity that it wasn't dragged the 100 metres or so to the roundhouse at Goulburn for the "Streamliners" display as the last 930 class in traffic and the last on standard gauge.