The simple fact is this, when you re-register any vehicle in NSW the authorised inspection agent only looks a two numbers, the chassis and engine number. If its only been out of rego for less than 3 months they will just look at the old rego papers. If more than 3 months they need to check the real numbers and give you a blue slip. They then base the identity of the car on thoses numbers. That is why the tags do not matter.
The body really could be anything from a HQ sandman van shell to a Nissan Patrol. It will still show on the rego papers as what ever the chassis came out of.
If they had given the chassis a part number and stamped the chassis number on the firewall, it would be as Two Tees states. Then the body and tags would be the identifier. They did not though, so the tags AND the chassis are the vehicle. the tags on their own are as good as the tags off an A9X Torana without the body, interesting, but incomplete.
The tags without the chassis identify what the vehicle was, not what it is.