ERSA states:
Airways clearance requests to enter SY CTR must be made on BANKSTOWN GROUND 119.9 during hours of activation of BK TWR
This would suggest to me that you do need to specifically request airways clearance. It is also possible to depart IFR OCTA, in which case you are still required to make a taxi call for the purpose of initiating SAR. So without the "request airways clearance" the controller may be unsure what you would like to do.