If you could find the legislation outling the legality of it, then that would help. An Officer does not have to prove the vehicle is unroadworthy to defect it. He/she only as to have cause to believe it is. It's then up to the driver/owner to prove to the Transport/Road/Vehicle Authority that the vehicle is roadworthy before you're vehicle can then be driven.
All of that can just be avoided by being able to show/state if it is legal to the cop. Remember, even coppers have grumpy days when they wil ticket and defect EVERYONE!
I'd look it up, but I've tried researching something in the ADR's before, and they are HUGE!