While it was premeditated. It was not premeditated murder. It was a preemptive strike!
If it was just the lovely lady, she could have walked. While to many women in her situation, "just walking" doesn't seem like an option, but it was. But her kids were involved. If she walks, he can dangle their offspring as a weapon. I wasn't at the trial, so I don't know, but it sounds like kill or be killed. Chances are I would have acquitted, given just this snipet of details from the case.
Granted, looking at this from the perspective of Australia's ideas of gun ownership, I would say the idiots from the gun control groups will point to this man as an example of why the proles shouldn't have guns, and pass right over the fact that this woman's only chance was the equalizer of a good rifle.