well because you seem to be in a panic, I will say the following first:
Dont worry about doing this because it is not something you ever need to do in HSC economics. You only ever really need to know that AD = C + I + G + X - M and that in equilibrium Y = AD.
Now what I think you are trying to do is destroy your brains by going through the ABS national accounts and try to see how they all add up. Dont bother, waste of tiem. First of all from an accounting sense GNI is not the same as GDP. Gross National Income refers to GDP plus interest and dividends from abroad. In this sense you might find some difficulties using the C + I + G + X - M method.
Otherwise if you are trying to calculate GDP, then C + I + G + X - M should work. But I dont know for the life of me why you are doing this. If you could give me some background on what you are trying to do perhaps I can give some better advice.
EDIT:
Upon second reflection, it could be that you are dealing with multiplier questions? I.e. a rise in autonomous investment will increase income by X? Anyway, give me some more information because it is not enough to answer your question.
