1. (a) That's right. It is important to notice that the events took place in Victoria. In New South Wales, South Australia and the ACT, such cases are governed by the general tort of Negligence and it is unlikely on the facts that A would be liable in Negligence.
In the other states and territories, there is a long-established tort of 'cattle trespass'. The advantage of cattle trespass over Negligence is that it imposes strict liability - a plaintiff need not prove that the defendant knew or ought to have known that their cattle might stray and cause damage. A would be liable to B for cattle trespass.
Cattle trespass makes the owner liable not only for damage to a neighbour's crops, but for any damage caused by the wandering cattle, such as passing on an infection, injury to persons, or injuring other animals.