How can they be bomb-proof? The # 1 most dangerous risk with any compressed flammable gas container is a BLEVE (Boiling Liquid Escaping Vapour Explosion) in fire fighting terminology.
If a tank is actually going to rupture because it's over-filled, then the tank isn't designed properly. I'm not sure if this is part of the Australian Standard for LPG tanks in motor vehicles, but the common-sense rule of thumb is that you design a tank to safely hold up to double it's rated capacity of gas to provide a suitable margin of physical safety.
But if the design is flawed, or the tank hasn't been made properly, or it's damaged as a result of an accident, it's an explosion just waiting to happen.
Anyone who was in Sydney on the night that the Boral LPG facility at Cooks River failed and several of the very large fixed tanks BLEVE'd will know just how powerful that sort of explosion can be. 8-)
Craig.