monetary policy is discretionary in that the RBA does determine what the i.r's will be. I suppose if u wanted to be all philosophical about it, u could say that interest rates that actual prevail in the market can be non-discretionary depending on bond yields etc...but thats not monetary policy per se...
So yeah, u would never have to refer to MP being discretionary/non-discretionary. The reason u have the distinction with fiscal policy is that some components of the budget result from what the government *chooses* to do in pursuing its own political/social/economic agenda, and some (e.g automatic stabilisers) come about as a result of economic conditions, that the government has not actually made a conscious decision to modify/alter revenue/spending there.