When I was growing up my father was heavily into restoring vintage cars (and still is) and I have many memories of sliding around in back seats of 1930's vehicles due to there being no seatbelts fitted. We also used to get pulled over regularly when out in our 1970 HG Brougham, as it too had no rear seatbelts - but there was nothing that the Police could do as it was ADR (Australian Design Rule) compliant.
As far as I am aware - and please someone correct me if I'm wrong - that if the vehicle was not originally built with seatbelts as specified on the compliance plate (i.e. ADR's), then they do not need to be retrofitted for registration.
If you were to fit seatbelts as Guyricho did, you would have to have it inspected and approved by an engineer as you are fitting 'non-standard equipment'. The vehicles compliance plate would have to be modified to reflect this, and you would be forever bound to have seatbelts fitted.
Cheers,
Andy
