Thanks Colin, I hadn't realised that! 
I've done a quick look around: the Brisbane Tmes 
said that mandatory belt wearing came into force in Qld in 1972, however other sources said it was mandatory in SA and Vic in 1971.
The best source that I found was 
here in this historical report. In particular, Table 6 on (paper) page 16 shows that the design rules for rear seat belts existed by January 1971, and the conclusion is very interesting as well. 
I know that Tallulah has been registered in the ACT for over 25 years, and I think she was in NSW before then. She clearly hasn't lived on the coast anywhere.[thumbsupbig]
Cheers, Mike.