Even as a commuter you will be expected to be in base on standby (there is a clause in the EBA allowing you to be out of base at company discretion). I’m not a Perth SO but I would imagine if a junior blank line holder you could expect 20+ Days standby in a roster if there’s no trips to do.
Before a trip they want you there 24hrs before departure, and at least one SO has been stood down for arriving too close to departure. Flights can be quite full and sometimes cancelled so always leave plenty of time to get to Perth and be adequately rested.