In NSW the rules are simple:
* If the car has never had them fitted and is otherwise original (or at least does not have sufficient modifications to warrant an engineers report) then you don't have to fit them.
* If the car is modified, was legally registered without them (ie a long time ago) and that rego has never lapsed, you don't have to fit them.
* If the car has ever had belts fitted, they must remain fitted. And of course, if they're fitted, they must be worn by the occupants
I imagine that VicRoads have similar doco you can obtain, but in NSW the rules regarding light vehicle modifications are very clear. If the increase in cubic capacity is less than 10% (ie 149 just scrapes in on the presumption that all greys are 138cid), then you don't need the car engineered. Any more than that and you do. "Getting the car engineered" includes meeting the guidelines set out in the "Light Vehicle Modifications" document you can download from the RTA's site. These basically boil down to: 2-speed wipers, demister, seat-belts (all the way around) and disc brake front, on top of the basic rego requirements, and anything the engineer tells you to do :-/ This is a bit more subjective, and depends upon whatever other mods you might have done. You can of course shop around for engineers if you think one you've spoken to is being unreasonable.
That's a long answer to a short question
cheers,
RET