Yup, that is correct NSW will base it on the Chassis number. HX chassis under a HQ body = HX
Same with engine, Put a blue/black motor in a HQ and you have to meet the ADR's of the blue/black engine.

That's not to say some dodgy things didn't get through. Like re-stamping numbers etc or confusing the girl at the DMT/RTA/TRMS counter!

When I replaced the passenger skirt on my 9/71 Bathurst Special XU-1 I wanted to weld in and retain the original chassis number and wasn't taking no for an answer.
I had to go to the NSW police with all documentation, fill out a stat dec and have the car and number inspected. The police wanted to add a police number. I said NO WAY. In the end they agreed to allow the original number and police documention stating it had been inspected, cleared and the reasons why.