Straight from CASA
"All flight time during which the aircraft was controlled solely by reference to instruments may be recorded in the instrument 'Flight' column:
a) Time above overcast or at night in Visual Meteorological Conditions (VMC) is not counted as instrument flight;
b) In actual or simulated instrument conditions, only the pilot manipulating the controls or providing input to the auto-pilot may log all flight time as instrument flight;
c) A flight conducted on an Instrument Flight Rules (IFR) flight plan is not to be counted as instrument flight unless flying in IMC;
d) Instrument approaches are to be credited to the pilot (pilots, in the case of an airborne radar approach) manipulating the controls or providing input to the auto-pilot during the approach. "
You can't count IFR fllight unless in IMC in the instrument flight column. Simple really. If everyone follows the rules then we all know what to expect from a given amount of Instrument time in the log book. I know its not prefect but at least its a standard.
Cheers