Queensland has "unique" rules. The apply Australian Design Rules (ADR) retroactively to require seat belts. it is, in reality, a joke as you need an "engineer" (e.g. someone who has worked on cars a long time) to approve. I spoke to at least a dozen, all had unique requirements.
For the coupes/fastbacks it pretty straightforward, apply a 3 point system. Some allow use of the factory mount when available, other require welding in plates, as its hidden not a big deal.
For the convertible the fix is to mount the belt to the rear of the door jamb. A bad "solution", but in reality the only realistic option short of adding a structural roll bar.
For the rear the shoulder mount was tacked to a bracket bolted to the seat back support.
As my '68 S/C6 is kinda unique I did not want to cut/weld and shopped around until I found a guy who approved a "bolt-in" option (did require drilling a few hidden holes, which I loathed doing but had no choice).
End result, the reel is behind the rear 1/4 inner panels and it will get registered. Most just mount then in the footwell and IMO it looks like crap. Once it is registered, they come out and go back to stock (as if the car was here 10 years ago and registered none of this would have been required as it would be gradfathered in...). IMO there is no real safe way to add 3 points to a early convertible if you are over 6' tall (I'm 6'2"). The fix would be rejected in a heartbeat if any new car proposed such a dodgy "fix"...