My cat was killed by a dog ( australia )?

The gate on the neighbors house was left open, and the dog came over to my house and mauled my cat to death. i ordered a report from the vet to describe the damage done to her. I am concerned because i have a 4 year old son that plays outside in our yard, and im scared the dog might get loose again and try to attack him. I am new to australia and i know the rules are different here on prosecuting. thanks.