G'day,
in Vic there is some obscure Law that hints that any REPLACEMENT seatbelt fitted to a car must carry the tag that lists the Australian Design Rule to which it complies.
If your car arrived in australia with lap only belts, then they are passable. If they are shagged and you wish to replace them, then they must carry the ADR tag....good luck finding some!
This is REALLY an obscure area. (brain working overtime)
As it stands from an insurance point of view. If the belts are as fitted to factory...i.e in your case lap only, then they are happy. If you have to go through an engineer, then the Registration Mob and your insurance will honour whatever the engineer signs on.
You don't mention if your car is registered or not. If it is registered then you won't have too much problem at all..legally as you have 'improved' or 'updated' the safety level of your vehicle.
Possibly the biggest issue you face is are you actually going to improve the safety of your car?
A three point belt incorrectly mounted is actually capable of doing more damage than a lap belt. A lap belt, you may smash your snozz on the wheel, but an incorrect 3 point will break your back.
If you are serious, and want to do it right! consult an engineer, if not to sign on it, at LEAST for good sound advice on the quality of the kit, and getting the mount hieghts correct.
Cheers
Jas