Excellent question!
And what a difficult one.
I guess the owners are at fault however having lived on 5 acres with dogs and llamas as well as smaller animals, this was sometimes difficult.
People would leave the front gates open and so the dogs got out -occasionally there was a problem with the fencing which we were not aware of until a dog was out.
I know this is all the owner's responsibility however I do not think it is fair to just shoot a dog without a warning.
And also without proof or photo of the event -I would rather report it, with the photo to the ranger and let the council decide.
I understand that there are problems with this as what if an animal is killing a whole lot of farm animals? I think then perhaps you need to take steps to stop him but not to shoot him dead.
As I said, it is a very difficult question and depends which side you are coming from, and each situation is different.
I am sure however that farmers on really large properties do this anyway and no one would know what has happened to their dog.
Either way, it is a horrible thing....