I can't believe somewhere the size of Sydney can't supply a M12 x 1.5 bolt. I walked into Suffolk Fasteners in Ipswich and bought one over the counter. As an aside, does anybody know why bolts are used instead of studs? Every British car I've owned up to the 1980s had studs, but the French seemed to favour bolts. Is there a technical reason?
I think you'll find that as in most cases with the motor manufacturers, it comes down to cost! Yes they can save fractions by just drilling and tapping holes in the hub and providing bolts to fasten the wheels on. To have the original and much better system involves drilling holes in the hubs and then machining splines in them, then making studs with splines on the inner end, and these have to be accurate for an interfernce fit, then fitting the studs into the hubs, and then producing wheel nuts with the right taper. So much more expensive.
But fitting wheels, holding them in place, trying to align the holes and then using wheel bolts is a bl***y pain! So the best way is to carry a mandrel which you can screw in one hole, slide the wheel on that and fit three bolts, then replace the mandrel with the final bolt. I made a mandrel some years ago with an M12x1.25 thread one end and an M12x1.50 thread the other end and it can be used on either type fitting.
FIAT used to actually provide an M12x1.25 mandrel in their tool kit, to make fitting the wheels on their cars easier.
Roy