They can only void the part that they can prove responsible for the damage. For example, your suspension mod may void the suspension part of the car's warranty but not the drive shaft, clutch or the engine.
If you tune the engine, may have void the warranty of the engine. But you are potential to void the clutch, gearbox or the driveshaft, if they can prove it the extra power causes the damage if not the faulty of the part itself.
For any make, the dealer will not try to void your warranty. The amount they charge to you or the head office, in this case, BMW Australia, is the same. There is no benefit to them beside losing a customer to blame on the mod and not trying to claim warranty for you, unless they cannot clearly write in their claim form about the faulty part. At the end of the day, it is the head office to decide the claim is valid or not.