Yes, you do call Flightwatch with your amended plan ASAP after your diversion. It is part of your SARTIME that if something goes wrong AusSAR do know where you have headed of to. It is also for traffic reasons. Your position and intents are passed on to IFR traffic as known VFR.
Just get back up there an re-do your knowledge deficiency and don't forget to grill your instructor for not showing you or at least talking to you on diversion procedures.