To do a co-ordinate shift in Civil3D the easiest way is to do a move from the Arbitrary Point to the MGA zone point is to use the AutoCAD command MOVE.
Before doing the move you will need to do the following
Go to the Options
Click on the Drafting Tab
Go to the Object Snap Options
Tick ON replace Z value with current elevation. (If you don't tick it ON the Z values will change when you snap from the Node of one point to the Node of another)
Now go back to the drawing and select everything in the drawing other than the correct MGA point that has been added, move all the points from the Arbitrary points shot in the field as the MGA point to the MGA point itself. It is now on a co-ordinate datum.
Level Datum Shift
To shift all points in the drawing from an arbitrary datum to a level datum we do the following:
Click on Points at the top or Point Groups - All Points
Pick one point down the bottom and then do CRTL A and all points will be highlighted Blue
Right click - DATUM. You need to minus the amount required to get the right level e.g. PM is 56.258 your Arbitrary level is 100 so 100 - 56.258 = -43.742. So when it asks for the change in elevation you need to take away 43.742 to get the 56.258.
Now you have the Co-ordinates on MGA as well as the Levels on the correct Level datum.