for govt authority scripts which require a phone call to the authority line for the script to be authorized, they will find out as soon and the person on the other end of the line speaks to them matey
PS refer above about what i said about the DDU. medicare has nothing to do with it. they just fund shit.
The OP never said he was doctor shopping or had obtained his script illegally. The DDU (in QLD and SA, different names in the other states) is mainly involved in making sure this is not happening, and yes they do handle benzodiazepines. If the OP were handing in scripts from more then one doctor, then the DDU might get involved.
But he simply has two scripts from his doctor; one is for diazepam the other for sertraline. He simply wants to get the diazepam and not the sertraline. No flags raised or anything; the pharmacy might question him about why he isn't filling the other script but the DDU would have nothing to do with it really. He didn't obtain it illegally as far as we are aware.
Diazepam, whilst it is considered a potential drug of dependence, is very commonly prescribed. You should keep this in mind. Him handing in one script for it from a doctor is far from what raises questions with the DDU.
Also, authority script doesn't just mean that it's a highly abusable drug that has to be authorized by the authorities; many other substances are authority scripts. It means that for them to be covered under the PBS, they have to be used for certain indications and authorized for that.
See the other thread regarding ER doctors where I have already discussed this; believe me, I know how this works. Medicare can be contacted to see if they have paid for a script, and Medicare can and does sometimes pick up on doctor shopping and report it.
I didn't go into this last night as I was drunk, and I am hungover this morning so this may be all over the place. If you need anymore clarification, ask me.
ETA: Besides which, diazepam may even be a streamlined authority script, requiring no calls to Medicare to authorize it's use prior for the required indications before the script is written.
ETA II: Diazepam isn't even an authority required script unless it is being used for palliative care.