Roadside Driver Drug Testing - how samples are managed

What is the main reason for wanting the drug swipe back? It is DNA collection/databasing concerns?

Yes and also having control over what belongs to me.
With regards to the process being anonymous, when you are pulled over the Police have already taken a photo of your car rego plates and run it through the system. So they already know who the car should belong to. Then they ask you for your license which shows who you are. So information has already been collected (for good or bad)

I’ll second that question; what do you need to be prepared for in this regard?

I realise that the Police officers at the testing unit are acting in accordance with the Standard Operating Procedures that states that they do not hand back Drug Swipe Tests,
but those procedures do not have any legal backing. So I would have liked to have a letter from the Police minister or Commissioner stating that it was alright to hand a negative result back to it’s owner.