The BMW is not responsible for hitting the car in front! that only happens if the cars are moving, then it's his fault for not keeping enough gap.
If the cars are stationary, nothing is happening. Your gf goes boom. she caused the damage to both.
Although 12k does seem a bit much, it'd be difficult to not pay. See, the thing is, your gf did the damage. The guy got it fixed. I mean, the panel shop probably went oo, a bmw, lets reem the insurance company so they charged 12k.
The difficulty is this. You are just a normal person. NRMA is a massive company. They'd be willing to fight you to the death for it. Can you afford legal advise? a good lawyer at 250+ an hour? Even if you win, you don't get all your costs back.
You might just have to pay, and take it as a lesson learnt
Lesson learnt: don't drive a car without insurance. Definitely, don't let a gf drive your car, especially if it deosn't have insurance.